add initial checked state on TreeItem

[yopiyop]

with the following change to TreeBinder :



public void setCheckedProperty(String checkProp) {
this.checkProp= checkProp;
}

protected TreeItem createItem(ModelData model){
...
if (model.get(checkProp) !=null ) {
item.setChecked((Boolean)model.get(checkProp));
}
...
}