…from priv->button. My refactor to g_signal_disconnect_by_data()
included this widget, when I shouldn’t have as both modes use it.
This e.g. broke opening a CB by keyboard that was currently in menu
mode, if it had been in list mode initially (e.g. due to the theme).

Fix by moving to disconnect_by_func() and only removing in each mode’s
destroy() method the signals that it set on the button in its setup().

https://bugzilla.gnome.org/show_bug.cgi?id=788577