ui/cocoa: Subclass NSApplication so we can implement sendEvent

ui/cocoa: Subclass NSApplication so we can implement sendEvent

When we switch away from our custom event handling, we still want to
be able to have first go at any events our application receives,
because in full-screen mode we want to send key events to the guest,
even if they would be menu item activation events. There are several
ways we could do that, but one simple approach is to subclass
NSApplication so we can implement a custom sendEvent method.
Do that, but for the moment have our sendEvent just invoke the
superclass method.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Tested-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-id: 20190225102433.22401-7-peter.maydell@linaro.org
Message-id: 20190214102816.3393-7-peter.maydell@linaro.org