gtk: implement -full-screen

gtk: implement -full-screen

Aiming for GTK as replacement for SDL, a feature like -full-screen should also
be implemented.

Bringing the window into full-screen mode is done by activating the "Fullscreen"
menu item. This is done after showing the windows to make the cursor and menu
hidden.

v2: drop -no-frame implementation, use booleans instead of ints and ensure
    consistency between ui state and menu.

Signed-off-by: Peter Wu <lekensteyn@gmail.com>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>