Start reimplementing ViewTree.

CCBUG:337516

What works:
- Basic mouse/wheel/keyboard interaction.
- Context menu.
- Keyboard event forwarding to views.
- Spring loading on drag hover.
- Notification colors (no icons yet, the led engine is unported).

What doesn't:
- Not actually a tree yet.
- Custom delegate appearance.
- Seperator item.
- DND.
- Close buttons.
- Middle click close.