background: Load bg data when selected

And only when selected. There's no point in loading it straight
away, as we have more than enough metadata to display it in the
icon view, and we'll need to load up the data when we display
it anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=641414

panels/background/bg-pictures-source.c

@@ -244,7 +244,6 @@ file_info_async_ready (GObject      *source,
           item = cc_background_item_new (uri);
           g_free (uri);
           g_object_set (G_OBJECT (item), "flags", CC_BACKGROUND_ITEM_HAS_URI, NULL);
-          cc_background_item_load (item, info); /* FIXME use asynchronous load, and remove if failed */
 
           if (cc_background_item_get_placement (item) == G_DESKTOP_BACKGROUND_STYLE_NONE)
             g_object_set (G_OBJECT (item), "placement", G_DESKTOP_BACKGROUND_STYLE_ZOOM, NULL);