Recreate the GTK2 selected treeview graphic in GTK3.

Modifies row:selected:focus to recreate the GTK2 selected treeview graphic in GTK3.

This brings the old GTK2 style inline with GTK3, and supercedes pull #30.

GTK3 (Nemo):
![GTK3 Nemo](http://i.imgur.com/0HHU4gk.png)