As I didn't like available extensions, I've extended Ext.field.Select widget and added the multiselect feature. Works as a single or multi select field.
It can easily extend Ext.field.Text if to take the missing code from Ext.field.Select and add it to Ux.field.Multiselect.
Github repository: https://github.com/vadimpopa/Ux.field.Multiselect
Works on ST 2.x
Multiselect field example image:
Screen Shot 2013-03-14 at 7.09.29 PM.png
EDIT: usePicker config won't work as it is always used a list on both tablets and phones