Install .desktop file in /usr/share/xsessions, so that it will be recognized by
display managers as a valid session (Closes: #579054).

Index: tritium/setup.py
===================================================================
--- tritium.orig/setup.py	2011-12-21 13:46:26.729413387 -0600
+++ tritium/setup.py	2012-07-05 17:57:48.753749648 -0600
@@ -37,7 +37,7 @@
        packages         = ['tritium', 'tritium.frame'],
        scripts          = ['bin/tritium'],
        data_files       = [ ('/etc/X11/tritium/', ['etc/keys.py', 'etc/layout.py']),
-                            ('share/applications', ['extras/tritium.desktop']),
+                            ('share/xsessions', ['extras/tritium.desktop']),
                             ],
 
 # why, oh why, can't i make this work
