workspace: Sort windows in overview grid using cached bounding box

When accessing properties on ClutterActor for size and position there is a notable access time overhead. This overhead adds considerable user lag when opening the overview if many windows are open as these properties are accessed to sort open windows by.

Since the needed properties are already locally a part of WindowClone, those can be used instead when sorting to remove this overhead and improve performance.

In my experience, this has resulted some very substantial performance gains when opening the overview (or grabbing a window in the overview) with many windows open - something like 100 times better performance without exaggerating.