fontchooser: Fix objects-finalize test

fontchooser: Fix objects-finalize test

The treeview is evil and keeps reference cycles in the
form of various tree row references. That gets cleaned up
if you explicitly gtk_widget_destroy the treeview. But since
07f2024bfca22331121, the scrolled window no longer destroys
its child, exposing this issue as a reference leak in
the objects-finalize test.

The font chooser widget is affected here because it calls
gtk_tree_view_scroll_to_path from init(), which creates one
of those reference cycles. Work around this in the font
chooser by unsetting the tree view model in dispose, which
clears up this cycle.