ui/cocoa.m: prevent stuck command key when going into full screen mode

ui/cocoa.m: prevent stuck command key when going into full screen mode

When the user pushes Command-F in QEMU while the mouse is ungrabbed, QEMU
goes into full screen mode. When the user finally releases the command key,
it is sent to the guest as an event. The makes the guest operating system
think the command key is down when it is really up. To prevent this situation
from happening, we simply drop the first command key event after the user has
gone into full screen mode using Command-F.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 20180703020017.1032-1-programmingkidx@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>