Auto-close: fix possible crash at exit

Autoclose plugin uses plugin_signal_connect to connect to Scintilla
widget. This may cause a crash on old glib because widget is a short-lived
object. Now behavior changed: autoclose uses raw g_signal_connect call
and calls g_signal_handler_disconnect manually. But this may cause a
crash with old Geany versions and works good only with last revision of
Geany.