I'm happy to announce more one plugin for GXT 
look the attachment
Licence: GNU GPL license v3
Code
Code:
package com.extjs.gxt.samples.explorer.client;
import java.util.List;
import com.extjs.gxt.ui.client.Style.Scroll;
import com.extjs.gxt.ui.client.core.El;
import com.extjs.gxt.ui.client.data.ModelData;
import com.extjs.gxt.ui.client.data.PagingLoader;
import com.extjs.gxt.ui.client.store.ListStore;
import com.extjs.gxt.ui.client.store.StoreListener;
import com.extjs.gxt.ui.client.util.BaseEventPreview;
import com.extjs.gxt.ui.client.widget.CheckBoxListView;
import com.extjs.gxt.ui.client.widget.LayoutContainer;
import com.extjs.gxt.ui.client.widget.ListView;
import com.extjs.gxt.ui.client.widget.form.ComboBox;
import com.extjs.gxt.ui.client.widget.form.ListModelPropertyEditor;
import com.extjs.gxt.ui.client.widget.toolbar.PagingToolBar;
import com.google.gwt.dom.client.InputElement;
import com.google.gwt.user.client.Element;
import com.google.gwt.user.client.ui.RootPanel;
public class XComboBox<D extends ModelData> extends ComboBox<D> {
private String valueFieldSeparator = ";";
private String rawSeparator = ", ";
public XComboBox() {
messages = new ComboBoxMessages();
setView(new CheckBoxListView<D>());
setPropertyEditor(new ListModelPropertyEditor<D>());
monitorWindowResize = true;
windowResizeDelay = 0;
initComponent();
setTriggerAction(TriggerAction.ALL);
}
private void bindStore(ListStore<D> store, boolean initial) {
if (this.store != null && !initial) {
this.store.removeStoreListener(getStoreListener());
if (store == null) {
this.store = null;
if (getView() != null) {
getView().setStore(null);
}
}
}
if (store != null) {
this.store = store;
if (store.getLoader() == null) {
setMode("local");
}
if (getView() != null) {
getView().setStore(store);
}
store.addStoreListener(getStoreListener());
}
}
@Override
public void collapse() {
super.collapse();
String text = "";
for (D d : getSelection()) {
if (text.length() > 0) {
text += rawSeparator;
}
text += d.get(getDisplayField());
}
setRawValue(text);
updateHiddenValue();
}
private void createList(boolean remove, LayoutContainer list) {
RootPanel.get().add(list);
setInitialized(true);
if (getPagingToolBar() != null) {
setFooter(list.el().createChild("<div class='" + getListStyle() + "-ft'></div>"));
getPagingToolBar().setBorders(false);
getPagingToolBar().render(getFooter().dom);
setAssetHeight(getAssetHeight() + getFooter().getHeight());
}
if (remove) {
RootPanel.get().remove(list);
}
}
@Override
protected void doForce() {
return;
}
private native int getAssetHeight() /*-{
return this.@com.extjs.gxt.samples.explorer.client.XComboBox::assetHeight;
}-*/;
private native BaseEventPreview getEventPreview() /*-{
return this.@com.extjs.gxt.samples.explorer.client.XComboBox::eventPreview;
}-*/;
private native El getFooter() /*-{
return this.@com.extjs.gxt.samples.explorer.client.XComboBox::footer;
}-*/;
private native InputElement getHiddenInput() /*-{
return this.@com.extjs.gxt.samples.explorer.client.XComboBox::hiddenInput;
}-*/;
public String getRawSeparator() {
return rawSeparator;
}
@Override
public List<D> getSelection() {
return ((CheckBoxListView<D>) getView()).getChecked();
}
private native StoreListener<D> getStoreListener() /*-{
return this.@com.extjs.gxt.samples.explorer.client.XComboBox::storeListener;
}-*/;
@Override
public D getValue() {
return null;
}
public String getValueFieldSeparator() {
return valueFieldSeparator;
}
@SuppressWarnings("unchecked")
@Override
protected void initList() {
ListView<D> listView = getView();
if (listView == null) {
setView(new CheckBoxListView<D>());
}
String style = getListStyle();
listView.setStyleAttribute("overflowX", "hidden");
listView.setStyleName(style + "-inner");
listView.setStyleAttribute("padding", "0px");
listView.setSelectOnOver(true);
listView.setBorders(false);
listView.setStyleAttribute("backgroundColor", "white");
listView.setSelectStyle(getSelectedStyle());
listView.setLoadingText(getLoadingText());
if (getTemplate() == null) {
listView.setDisplayProperty(getDisplayField());
} else {
listView.setTemplate(getTemplate());
}
setAssetHeight(0);
LayoutContainer list = new LayoutContainer() {
@Override
protected void onRender(Element parent, int index) {
super.onRender(parent, index);
getEventPreview().getIgnoreList().add(getElement());
}
};
list.setScrollMode(Scroll.NONE);
list.setShim(true);
list.setShadow(true);
list.setBorders(true);
list.setStyleName(style);
list.hide();
assert store != null : "ComboBox needs a store";
list.add(listView);
setList(list);
if (getPageSize() > 0) {
PagingToolBar pageTb = new PagingToolBar(getPageSize());
pageTb.bind((PagingLoader) store.getLoader());
setPagingToolBar(pageTb);
}
if (!isLazyRender()) {
createList(true, list);
}
bindStore(store, true);
};
private native void setAssetHeight(int assetHeight) /*-{
this.@com.extjs.gxt.samples.explorer.client.XComboBox::assetHeight = assetHeight;
}-*/;
private native void setFooter(El footer) /*-{
this.@com.extjs.gxt.samples.explorer.client.XComboBox::footer = footer;
}-*/;
private native void setInitialized(boolean initialized) /*-{
this.@com.extjs.gxt.samples.explorer.client.XComboBox::initialized = initialized;
}-*/;
private native void setList(LayoutContainer list)/*-{
this.@com.extjs.gxt.samples.explorer.client.XComboBox::list = list;
}-*/;
private native void setMode(String mode)/*-{
this.@com.extjs.gxt.samples.explorer.client.XComboBox::mode = mode;
}-*/;
private native void setPagingToolBar(PagingToolBar pageTb)/*-{
this.@com.extjs.gxt.samples.explorer.client.XComboBox::pageTb = pageTb;
}-*/;
public void setRawSeparator(String rawSeparator) {
this.rawSeparator = rawSeparator;
}
@Override
public void setSelection(List<D> selection) {
for (D d : selection) {
((CheckBoxListView<D>) getView()).setChecked(d, true);
}
super.setSelection(selection);
}
@Override
public void setValue(D value) {
return;
}
public void setValueFieldSeparator(String valueFieldSeparator) {
this.valueFieldSeparator = valueFieldSeparator;
}
private void updateHiddenValue() {
if (getHiddenInput() != null) {
String v = "";
for (D d : getSelection()) {
if (v.length() > 0) {
v += valueFieldSeparator;
}
v += d.get(getValueField());
}
getHiddenInput().setValue(v);
}
}
}