The general idea here is that we need the masks to handle mouse down events, as they need to handle the drag (mousedown -> drag immediately).
I've rewritten the editor selections to use events, as there are some 3 different components that handle/trigger selections in different ways.
1. All selections/deselections now propagate through `HitObjectMask.Select()`/`HitObjectMask.Deselect()`.
2. Components that react to changes in the selection bind to the masks' `Selected`/`Deselected` events, and track them/change their states locally.
3. Masks provide a `SingleSelectionRequested` event which is invoked on the mouse-down event. Various components bind to this event to perform state changes locally in this scenario.
4. `DragBox` now handles all drag input locally. It triggers `Select`/`Deselect` on the masks it needs to.
5. `SelectionBox` handles the display of itself locally.
6. `SelectionBox` handles movement of groups of masks locally.
7. `HitObjectMasks` handles movement of itself locally.
