The point of this renderer is to draw an expander manually, thereby allowing
the tree view's expander to be hidden. This makes the contact appear at the
same indent level as the group. But if we're re-designing the whole widget,
there's probably not going to be a need for this manual work.

And it drops another file we'd need to port to GTK4.

Compiled and opened the buddy list; expanders were still there on groups. The contacts were just indented a bit more.