treeview: use gtk_tree_view_ensure_background() consistently

treeview: use gtk_tree_view_ensure_background() consistently

In gtk_tree_view_state_flags_changed() we were setting the background on
the bin_window without the necessary "view" style class, making the
treeview render with the wrong color in some circumstances (such as when
adding an empty treeview in a window, as spotted by Brian Smith).