meta-plugin-manager: Only allow one plugin to be loaded

The "multiple plugins loaded at once" strategy was always a big fiction:
while it may be viable if you're super careful, it's fragile and requires
a bit of infrastructure that we would be better off without.

Note that for simplicity, we're keeping the MetaPluginManager, but it only
manages one plugin. A possible future cleanup would be to remove it entirely.

https://bugzilla.gnome.org/show_bug.cgi?id=676855

src/compositor/compositor.c

@@ -557,9 +557,7 @@ meta_compositor_manage_screen (MetaCompositor *compositor,
 
   clutter_actor_hide (info->hidden_group);
 
-  info->plugin_mgr =
-    meta_plugin_manager_get (screen);
-  meta_plugin_manager_initialize (info->plugin_mgr);
+  info->plugin_mgr = meta_plugin_manager_new (screen);
 
   /*
    * Delay the creation of the overlay window as long as we can, to avoid