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/core/main.c

@@ -373,7 +373,7 @@ on_sigterm (void)
  * meta_init: (skip)
  *
  * Initialize mutter. Call this after meta_get_option_context() and
- * meta_plugin_type_register(), and before meta_run().
+ * meta_plugin_manager_set_plugin_type(), and before meta_run().
  */
 void
 meta_init (void)