Editor: fix disabling modifying actions for read-only editors

Now correctly disables all modifying actions and shortcuts of
texteditoractionhandler if the editor is readonly.

Task-number: QTCREATORBUG-4774
Change-Id: Ic212a2fe3b6dcc18e2fbc461d24bed031c67f6a2
Reviewed-on: http://codereview.qt.nokia.com/7


Reviewed-by: Bill King <bill.king@nokia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>