ui/cocoa: Don't call NSApp sendEvent directly from handleEvent

ui/cocoa: Don't call NSApp sendEvent directly from handleEvent

Currently the handleEvent method will directly call the NSApp
sendEvent method for any events that we want to let OSX deal
with. When we rearrange the event handling code, the way that
we say "let OSX have this event" is going to change. Prepare
for that by refactoring so that handleEvent returns a flag
indicating whether it consumed the event.

Suggested-by: BALATON Zoltan <balaton@eik.bme.hu>
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: 20190225102433.22401-6-peter.maydell@linaro.org
Message-id: 20190214102816.3393-6-peter.maydell@linaro.org