Source selector improvements

* Remove unwanted binding to cx column widgets - source selector
  is no longer shown in translation view
* Add a little bit of box shadow to the source selector dialog
* If the type is dialog, avoid a check for overlay shown or not. For
  dialoge overlay is always shown
* Update the hooks documentations size mw.cx.source.select is removed

Change-Id: I5ff598256813f0e22bfae826223b42b139a2dfec