Add GtkTreeExpander

Add GtkTreeExpander

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.