Move window quad management to scene items
The scene items depend on the scene windows for caching window quads. The goal of this change is to move window quads management to item.
Merging window quads in one list and then splitting them is inefficient, it will be highly desirable if window quads are removed from the public api so we can optimize window quad management.
With this change, the window quad type becomes irrelevant to render backends for the most part. Note that the Xrender backend is a bit nitpicky about window quads, so the shadow item doesn't create generic "WindowQuadShadow" quads anymore.
Unfortunately, I don't know a way around how to move forward with the scene redesign and not break the libkwineffects api. On the other hand, I don't want scene improvements be blocked by effects.
cc #30