Use more actor.grab_key_focus() and less stage.connect('key-press-event')

Until recently, the clutter keyboard focus was almost always kept on
the stage, and bits of code that wanted to do stuff with the keyboard
would just watch for key-press-events on the stage. In several places,
the code wasn't even bothering to ensure that the focus was on the
stage, which caused problems with other actors that explicitly grabbed
focus.

A previous fix for this (f21403fd) was to always reset the focus to
the stage after calling pushModal(), but a better fix is to just
actually make use of the keyboard focus everywhere rather than having
everyone try to read events off the stage.

Now pushModal(actor) also does actor.grab_key_focus(), and various
bits of code have been changed to read key events off their own
toplevels rather than off the stage, meaning there's no chance of them
accidentally getting someone else's events.

https://bugzilla.gnome.org/show_bug.cgi?id=618885