TODO:
- navigation
- expand/collapse subtree
- shift subtree up/down
- edit node
- multiple node selection
- create node
- mouse controls
- touch controls

Controls:
	up         - focus node above
	down       - focus node below
	left       - focus parent node
	right      - focus first child node
	tab        - indent node
	s-tab      - deindent node
	s-left     - collapse node
	s-right    - expand node