Refactor ext.translate.groupselector

So that I can:
* position it nicely
* set the language via options, not html attributes

For that I needed:
* to add jquery.ui.position of a current version, since core
  has very old version which doesn't support offsets like top+10
* make prepareMessageGroupSelector as member method to access 'this'

Change-Id: I18ffdae41d1833bf863d70b99613d20d2158750e
Merge "Refactor ext.translate.groupselector"