Only trigger grab sequence upon release

GTK-VNC uses the keyboard shortcut ctrl+alt to toggle inside/outside
the VM (aka release pointer). Now Gnome uses ctrl+alt+arrow_key to
switch virtual desktops. So these two shortcuts collide and gtk-vnc
will grab the key presses as soon as ctrl+alt is pressed and toogle
inside/outside the VM, which makes switching desktops impossible.

The solution is to only trigger the grab sequence when the user
releases the key. So if they press Ctrl+alt and then release it,
we trigger, but if they press Ctrl+alt+left-arrow we don't trigger

https://bugzilla.gnome.org/show_bug.cgi?id=685257



Signed-off-by: Daniel P. Berrange <berrange@redhat.com>