The main motivation behind this change is to share rendering code
between windows and the cursor, specifically the Item class which
requires a Scene.
Note that Scene subclasses are responsible for issuing
ItemRenderer::renderItem() calls. The main reason for that is the
current architecture of the effects system, specifically we need to call
some effects hooks before and after painting a window.
This is needed to establish explicit connection between an item and the
scene it belongs to. For now, the scene must be known at the item
construction time. Perhaps it can be improved in the future by items
inheriting their scene from the parent item, but the scene would need to
be refactored more so there's a root item or something like that.
