- Add single-key shortcuts when the preview is focused: F (fit window
  size), R (toggle recursively), +/- (zoom in/out), E/C (expand/collapse
  all), T (toggle the highlighted node), ? (shortcut help)
- Sync the state of toolbar buttons with the shortcuts
- Toggle a node by clicking anywhere on it, not only the circle;
  Ctrl/Cmd+click inverts the toggle-recursively mode
- Fix the 'Toggle the active node' commands, which never worked because
  the active node was not tracked