expression: Invalidate bindings before destroying them

expression: Invalidate bindings before destroying them

Use a weak ref to invalidate bindings. Make sure that this happens
before creating any watches, so that notifies from the
watched expression about changes will not trigger set_property() calls
during dispose()/finalize().

Invalidating also ensures that the watches aren't removed, which can
trigger warnings if the watches are watching the object itself, and the
weak refs cannot be removed anymore.