Cocoa: Fix qmlscene flicker on startup.

Cocoa: Fix qmlscene flicker on startup.

The SG render thread was racing window creation in
the GUI thread, which would cause flicker if the
window won the race and was shown before the SG
thread had a frame ready.

Send a synchronous expose event before showing the
window - this will wait for the SG render thread.

In addition, don't defer NSwindow creation. The GL
context setup is done before the window is shown
and needs a fully created window.

New API: QWindowSystemInterface::handleSynchronousExposeEvent
Retire: QWindowSystemInterface::handleExposeEvent

Change-Id: I0bb46089d16ec4882aaac8db67b57d15e0f51531
Reviewed-on: http://codereview.qt.nokia.com/3399
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@nokia.com>