This allows us to assume that the wrapper is always valid and remove
the _is_valid attribute.

One difference now is that g_boxed_free() is called when the wrapper can no longer
be used and that breaks the custom glib sources where g_boxed_free() triggers
the finalize callback which tries to call into a method of the wrapper.

To work around this provide a _clear_boxed() method which allows the wrapper
to free the underlying boxed in __del__ where the wrapper is still usable.