Cocoa: Merge QNSWindow and QNSPanel, inherit NSPanel

Since we will later have frameless NSWindow child QWindows, it makes
sense to be able to keep and reuse the instantiated NSWindows in cases
like QDockWidgets. If we keep these 2 different classes, it becomes
impossible to morph one into the other.

Task-number: QTBUG-33082
Change-Id: I351c628b887101b1cfd67689afbf689f119554de
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Move Direct2D paint engine on top of QPaintEngineEx

Make QWindowsDirect2DPaintEngine a subclass of QPaintEngineEx instead of
QPaintEngine like the other paint engines inside Qt. This should both be
faster and better supported, as the code paths handling QPaintEngineEx in
QPainter are tested more.

Additionally the visual output is closer to the raster engine, as all
primitives are drawn using the same mechanism, resulting in fewer
off-by-one-pixel errors.

This change also makes designer work mostly correctly, apart from some
remaining clipping bugs.

Change-Id: I53d36c08e73883ff35e389d57f9d30b482fc6721
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>