Shuffle things around in preparation for Direct2D platform plugin

Add a layer of abstraction in some spots where the Direct2D plugin will hook in.

Change-Id: Ifca7daf6ad0284af52a81822e970c2317d339234
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Add new direct2d platform plugin

This is an alternative plugin for the windows platform. It shares most
code with the current windows plugin, but substitutes a direct2d-based
paint engine for window backing stores and pixmaps.

Change-Id: I78fafd9c5871fa090b49436f5b40ec80f8789f8b
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>