Provide a D-Bus based StatusNotifierItem instead of a GtkStatusIcon

[aperezdc]

The current tray icon, which uses GtkStatusIcon is supported only under X11 because it uses XEmbed. On the other hand, KDE's StatusNotifier specification works over D-Bus and can work without X11. Many DEs support have varying degrees of support for StatusNotifier:

• Of course, KDE includes full support.
• XFCE via xfce4-sntray-plugin. This also works in Budgie/Solus, and with Vala Panel.
• Ubuntu has adopted the specification (here called AppIndicator instead, the D-Bus protocol is the same), and they work in Unity.
• Shamefully enough, GNOME has decided to not support the spec, though there is a (quite outdated) shell extension

There are a couple of options for implementing this:

• Ubuntu's libindicator, which does not seem to come with GObject-Introspection support, and makes it difficult to use from Python.
• @jjk-jacky's statusnotifier which is a pure C GObject library and GObject-Introspection support. It is even packaged in some distros, and it can always be bundled in Flatpak builds.
• Rolling our own implementation in Python which uses D-Bus directly.

I am leaning towards using statusnotifier.