toolbar: use finalize instead of dispose

We were disconnecting all the signals on dispose, except
the ones that comes from the widgets and are added with
gtk_widget_class_bind_template_callback. Therefore those
can be emitted after a dispose.

In the toolbar case, we connect to the toggle signal of a
button, which when the toolbar gets disposed, the buttons gets
untoggled and the signal is emitted, then the toolbar tries to
perform actions on external data that was cleared already on dispose.

To avoid that, just clear the data on finalize instead of
dispose.