Switches to the explicit maximized decoration element and animates
the button groups and caption.
Legacy support for just the centered element is still missing. Unsure
if it should be added or if it makes sense to break compatibility here.
What's working:
* background for active and inactive decorations inclusive transitions
* all buttons get loaded
* transition between button states
* all borders, paddings etc is working
* mouse interaction with buttons and title area
What's not yet working:
* special maximize mode
* mouse wheel on title area
* window tabs
* changing themes
* crash resistence (currently a not compiling QML file crashes KWin badly)
* window/blur mask (tricky - we need the alpha mask of the background SVG)
What's going to be dropped:
* special opaque mode
* decoration position at left/right/bottom instead of top
Why? Because nobody uses these features
