Enable DWM transparency for OpenGL windows under Windows

Starting with Windows Vista, we can control the compositing window
manager using the DWM APIs. This allows us to make truly transparent
OpenGL windows.

We also have to detect and listen if compositing is enabled, otherwise
enabling transparency will cause glitches all over the place.

This (partially) fixes
Task-number: QTBUG-28214

Change-Id: I0fe1ec7adec8181b788c32de03c59142731d9e7f
Done-with: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>