Installs a watch for the given `expression` that calls the `notify` function whenever the evaluation of `self` may have changed.
GTK cannot guarantee that the evaluation did indeed change when the
notify gets invoked, but it guarantees the opposite: When it
did in fact change, the `notify` will be invoked.
the `this` argument to watch
callback to invoke when the expression changes
user data to pass to the `notify` callback
destroy notify for `user_data`
The newly installed watch. Note that the only reference held to the watch will be released when the watch is unwatched which can happen automatically, and not just via [method@Gtk.ExpressionWatch.unwatch]. You should call [method@Gtk.ExpressionWatch.ref] if you want to keep the watch around.