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.
Merge pull request #31 from wrouesnel/gtktreeview-whitespaces-2

Recreate the GTK2 selected treeview graphic in GTK3.