shell: Add getter/setters for all properties

Traditionally, getter/setter functions have been considered a
C convenience, and we therefore didn't bother to add them for
many properties that are only consumed from JS.

However now that gjs optimizes property accesses by calling the
appropriate getter/setter instead, it makes sense to add them.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3620>

src/shell-window-preview-layout.h

@@ -19,6 +19,8 @@ struct _ShellWindowPreviewLayout
   ShellWindowPreviewLayoutPrivate *priv;
 };
 
+ClutterActorBox * shell_window_preview_layout_get_bounding_box (ShellWindowPreviewLayout *self);
+
 ClutterActor * shell_window_preview_layout_add_window (ShellWindowPreviewLayout  *self,
                                                        MetaWindow *window);