Fixes issue where `fcitx5` app icon persists after it is no longer
running. It seems it was getting a `NameOwnerChanged` event with the
acquired name after `register_status_notifier_item()`.
This is based on the GTK version of the status area applet that was
previously in this repository.
This exposes app indicators found over dbus. As used in applications
like nm-applet and steam.