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
Tested&works on ST 2.1 , 2.2
Multiselect field example image:
Attachment 42406
EDIT: usePicker config won't work as it is always used a list on both tablets and phones

