Cocoa: Post event to "show()" a modal window

The way we were doing it, we may have interferred with other
events in the CFRunLoop source and call processEvents() at
the wrong moment or for the wrong reason. By using a posted
event, we make the notification channel unambiguous.

This ammends ff3dcc49c4a1912189091e35e87cb61af2f62d47.

Task-number: QTBUG-38214
Change-Id: I94f7e89cf4c9803289749394f85119cba62ef0e7
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Doc: document obsolete QApplication::setGraphicsSystem()

Change-Id: I5a87564d2dbfa5e739d1ca40db480e8e0c1b7fdf
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Doc: document QTimeZone::swap()

Change-Id: I80d55df65546b671772ed93cb852e88ee040114a
Reviewed-by: Sze Howe Koh <szehowe.koh@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>