introspection: Build with --warn-all --warn-error

* Use --warn-all, --warn-error
* Fix various broken gtk-doc
* Drop unused shell_get_event_related
* For header defines, we currently require them to end in _H to be skipped
* Drop the no-longer-necessary fix-meta-rectangle.py hack
* Move to the convention of using -private.h for headers that are,
  well, private.
* Add shell-wm-private.h

src/st/st-texture-cache.c

@@ -831,7 +831,7 @@ st_texture_cache_bind_pixbuf_property (StTextureCache    *cache,
 }
 
 /**
- * st_texture_cache_load:
+ * st_texture_cache_load: (skip)
  * @cache: A #StTextureCache
  * @key: Arbitrary string used to refer to item
  * @policy: Caching policy
@@ -1272,6 +1272,8 @@ st_texture_cache_load_uri_sync (StTextureCache *cache,
  * This function synchronously loads the given file path
  * into a COGL texture.  On error, a warning is emitted
  * and %COGL_INVALID_HANDLE is returned.
+ *
+ * Returns: (transfer full): a new #CoglHandle
  */
 CoglHandle
 st_texture_cache_load_file_to_cogl_texture (StTextureCache *cache,