TODO:
- persistent empty first/last node (a button to create a new node)
- loading from DOM -- fill textarea
- Firefox compatibility -- remove ':has(..)'
- focus management
- mouse/touch controls
- navigation
- expand/collapse subtree
- shift subtree up/down
- create node
- edit node
- shifting nodes up/down
- multiple node selection
- undo 
  - delete node
  - indent/deindent
  - edit node
- copy/paste nodes/trees
- serialize/deserialize
- add optional text styling to nodes

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
	enter      - normal mode: edit node
	           - edit mode: create node below
	esc        - exit edit mode