Make treeview expanders work reliably

Make treeview expanders work reliably

It turns out that the gtk_grab_remove() can trigger a do_prelight()
call, which may end up changing prelight_node, and then the state
gets messed up. Moving the grab removal until after we're done with
button_pressed_node and prlight_node makes expanders work reliably.

One thing that is still not right is that the expander doesn't get
prelight again after the animation is done, if you manage to release
without any additional motion events.

https://bugzilla.gnome.org/show_bug.cgi?id=641039