manager: get all connected sources

Added ::get_sources_connected API to retrieve a list of every source the
application has connected, no matter whether they are enabled or not.
Use ::get_sources_connected on calendars menu in GcalWindow to enable
disabled sources.

src/gcal-window.c

@@ -776,7 +776,7 @@ on_calendar_toggled (GSimpleAction *action,
 
   /* lookup source */
   source = NULL;
-  l = gcal_manager_get_sources (priv->manager);
+  l = gcal_manager_get_sources_connected (priv->manager);
 
   for (aux = l; aux != NULL; aux = aux->next)
     {