treeview: Unify all line drawing into the same function
treeview: Unify all line drawing into the same function

This is useful because the dashing stuff needs to be done and it seems
like a good idea to unify it.

Unity-Technologies/gtk,johne53/MB3Gtk-2,Unity-Technologies/gtk,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Unity-Technologies/gtk,Distrotech/gtk,Unity-Technologies/gtk,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,Unity-Technologies/gtk