Improve direct2d backing store implementation

Add a per-window swap chain to a QPlatformWindow subclass instead of tying
it to the backing store. This is needed to support native widgets (as
opposed to alien widgets).

Change the backing store to draw to an intermediate pixmap and flush to
the requested window by using the per-window swap chain. This also opens
the door for faster window presentation later on by using the swap chain
more intelligently.

Also add a changelog entry for the direct2d plugin, which was omitted
earlier.

[ChangeLog][QtGui][Windows]
Introduce experimental direct2d platform plugin for Windows. This plugin
shares most code with the current windows plugin, but substitutes a
direct2d-based paint engine for window backing stores and pixmaps.

Change-Id: I5f54e7e4c1fb15b1639bd26b712fb40ac141e4ac
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>