gtk: add support for input grabbing (v2)

There is a small deviation from SDL's behavior here.  Instead of Ctrl+Alt
triggering grab, we now use Ctrl-Alt-g to trigger grab.

GTK will not accept Ctrl+Alt as an accelerator since it just consists of
modifiers.  Having grab as a proper accelerator is important as it allows a user
to override the accelerator for accessibility purposes.

We also are not automatically grabbing on left-click.  Besides the inability to
tie mouse clicks to an accelerator, I think this behavior is hard to discover
and since it only happens depending on the guest state, it can lead to confusing
behavior.

This can be changed in the future if there's a strong resistence to dropping
left-click-to-grab, but I think we're better off dropping it.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-id: 1361367806-4599-6-git-send-email-aliguori@us.ibm.com
