After a40f2b54, the button data is only
removed when button_data_free() gets called explicitly, which only
happens in two places: nautilus_path_bar_clear_buttons() and
button_data_file_changed(). What also happens is that buttons are
removed as part of their containing container destruction, which was
accounted for when a weak ref was used. Now that that never happens,
some leftover signal handlers wreak havoc.

Fixes GNOME/nautilus#593