gtk: add support for screen scaling and full screen (v5)

Basic menu items to enter full screen mode and zoom in/out.  Unlike SDL, we
don't allow arbitrary scaling based on window resizing.  The current behavior
with SDL causes a lot of problems for me.

Sometimes I accidentally resize the window a tiny bit while trying to move it
(Ubuntu's 1-pixel window decorations don't help here).  After that, scaling is
now active and if the screen changes size again, badness ensues since the
aspect ratio is skewed.

Allowing zooming by 25% in and out should cover most use cases.  We can add a
more flexible scaling later but for now, I think this is a more friendly
behavior.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Message-id: 1361367806-4599-7-git-send-email-aliguori@us.ibm.com