treeexpander: Implement input support

treeexpander: Implement input support

This implements all the keybindings from GtkTreeView that can be
supported.

It does not implement expand-all, because supporting that means
causing the TreeListModel to emit lots of create_model vfuncs which in
turn would cause many items-changed signal which in turn would cause
many signal handlers to run which in turn would make "expand-all" very
reentrant, and I'm uneasy about supporting that.

For the mouse, just add a click gesture to the expander icon that toggles
expanded state.