introspection: Build with --warn-fatal, drop fix-meta-rectangle.py hack

Cleanly build with --warn-fatal.  Implementation:

* Liberally apply (skip) where the API is clearly C only, e.g. uses
  XLib.  The theming code and MutterPlugin are skipped too.
* Add missing (transfer) and (element-type) annotations

For a few functions that had a comment, I turned it into gtk-doc, but
I didn't (with a few exceptions) try to write new documentation in
this pass.

src/core/keybindings.c

@@ -3547,6 +3547,10 @@ meta_keybindings_set_custom_handler (const gchar        *name,
   return TRUE;
 }
 
+/**
+ * meta_keybindings_switch_window: (skip)
+ *
+ */
 void
 meta_keybindings_switch_window (MetaDisplay    *display,
                                 MetaScreen     *screen,