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

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