Optionally install a support file for Xfce's Thunar file manager.
This adds an entry in Thunar's context menus to send files and
directories to the Audacious playlist. This is disabled by
default.

	modified:   Makefile
	modified:   configure.ac
	new file:   contrib/Makefile
	new file:   contrib/thunar-sendto-audacious-playlist.desktop
	modified:   extra.mk.in