ui/cocoa.m: verify with user before quitting QEMU

This patch prevents the user from accidentally quitting QEMU by pushing
Command-Q or by pushing the close button on the main window. When
the user does one of these two things, a dialog box appears verifying
with the user if he or she wants to quit QEMU.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 29169A74-0347-47F5-934F-A5AD24C225CA@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
