Readd gobjects.threads_init() in MPRIS frontend so that it may be imported before mopidy.core (e.g. by Sphinx autodoc)

mopidy/frontends/mpris/objects.py

@@ -7,6 +7,7 @@ try:
     import dbus
     import dbus.mainloop.glib
     import dbus.service
+    import gobject
 except ImportError as import_error:
     from mopidy import OptionalDependencyError
     raise OptionalDependencyError(import_error)
@@ -20,6 +21,7 @@ from mopidy.mixers.base import BaseMixer
 from mopidy.utils.process import exit_process
 
 # Must be done before dbus.SessionBus() is called
+gobject.threads_init()
 dbus.mainloop.glib.threads_init()
 dbus.mainloop.glib.DBusGMainLoop(set_as_default=True)