Currently we "only" grab the keyboard when starting a drag operation,
which does not impede keybindings to be processed. This is at best
not harmful (like workspace switching), but may have unintended effects
otherwise - for instance, the hot corner is disabled, so having the
corresponding keyboard shortcut still active is fairly odd (not to
mention that it leaves the system in a confused state).
Fix this by switching to pushModal()/popModal(), which will push a
dedicated keybinding mode for us.
https://bugzilla.gnome.org/show_bug.cgi?id=700877
9e56e668e0