password: Clean up signal handler in dispose

This is the only place where we connect to a driver signal,
and we never clean up the handler, so it is left dangling
when the pages get reconstructed (since the driver object
survives that). This might explain the mysterious memory
corruption crashes that various people have been seeing,
which usually involved this signal handler.

https://bugzilla.gnome.org/show_bug.cgi?id=733555
https://bugzilla.gnome.org/show_bug.cgi?id=732525