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.
It's quite a bit faster now, but the code is also a bit more awkward.
Pain points:
- GtkTreeListModel cannot be created in UI files because it needs
a CreateModelFunc.
Using a signal for this doesn't work because autoexpand wants to
expand the model before the signal handler is connected.
- The list item factory usage is still awkward. It's bearable here
because the list items are very simple, but still.
This is a container widget that takes over all the duties of tree
expanding and collapsing.
It has to be a container so it can capture keybindings while focus is
inside the listitem.
So far, this widget does not allow interacting with it, but it shows the
expander arrow in its correct state.
Also, testlistview uses this widget now instead of implementing
expanding itself.
