legacycontroller: Port to new API model

legacycontroller: Port to new API model

We no longer set the widget on construction, but instead require an
explicit call to gtk_widget_add_controller().

This way, the reference handling becomes explicit and bindable.

Because gtk_widget_add_controller() is (transfer: full), we don't
even need to unref the controller after adding it.
And we don't need to keep track of it, because controllers get cleaned
up by GtkWidget.