mpris: Only import 'indicate' if $DISPLAY is set

This avoids printing of the following error message:

    ImportError: could not import gtk.gdk

Fixes #311
mpris: Disable extension if no $DISPLAY set

We cannot connect to the D-Bus session bus if there is no X11 $DISPLAY
available.
Merge pull request #407 from jodal/feature/mpris-needs-display

Turn off MPRIS if no $DISPLAY available