Watch the lifetime of objects referenced in plugin->signal_ids and
remove our references to them if they get destroyed. This avoids
possibly trying to disconnect signals on destroyed objects when the
plugin is unloaded.
Supporting this case is safer, and is useful for objects that may or
may not outlive the plugin (like ScintillaObjects), because in such
cases plugin_signal_connect() is handy to make sure the signals are
disconnected if the object is still alive, but used to crash if the
object was destroyed.