qubes-desktop-run: don't crash on Debian wheezy (glib < 2.36)

Gio.DesktopAppInfo.get_boolean was introduced in glib 2.36. Instead of
crashing simply do not support DBusActivatable there. There is no such
application in default Debian wheezy template anyway.

misc/xdg.py

@@ -6,6 +6,7 @@ import dbus
 
 def launch(desktop, *files):
     launcher = Gio.DesktopAppInfo.new_from_filename(desktop)
+    if hasattr(launcher, 'get_boolean'):
         activatable = launcher.get_boolean('DBusActivatable')
         if activatable:
             bus = dbus.SessionBus()