scene: Split Scene class
Currently, the Scene class is used to represent the contents of the Workspace so rename it to match its usage.
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.