gtk: Hide the menubar when in fullscreen mode (lp 1294898)

gtk: Hide the menubar when in fullscreen mode (lp 1294898)

In fullscreen mode, we attempt to shrink the menubar to 1 pixel in height,
so it takes up as little room as possible while still allowing us to use
the keyboard shortcuts for its various operations.

However this shrinking is disregarded on gtk3, so the entire menu bar is
visible, which isn't very pleasant. This patch hides the menu bar instead.

The side effect is that the only keyboard shortcuts that will work in this
mode are the ones that we explicitly register on the top level window and
not the menu bar. The previous patches changed the fullscreen and vc
shortcuts to work like that, which I think are the only ones that really
matter in for the fullscreen case.

https://bugs.launchpad.net/qemu/+bug/1294898
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>