trayicon: migrate from libappindicator to org.kde.StatusNotifierItem

[deltragon]

This is necessary for migrating to gtk4, as gtk4 does not support libappindicator (neither the gtk nor ayatana version, see https://github.com/AyatanaIndicators/libayatana-appindicator/issues/22).

The org.kde.StatusNotifierItem is non-standard, however it seems more likely to be a way forward than libappindicator. See also https://pagure.io/fedora-workstation/issue/246 for a discussion on this. For example, this should also work on default ubuntu desktop installs, with GNOME using https://github.com/ubuntu/gnome-shell-extension-appindicator.

Unfortunately, since it is not standardized yet, there is no python library for using it, so this PR needs to create its own bespoke implementation of the DBus interactions. Additionally, this also drops dbus-python in favor of Gio's dbus.

This is tested in KDE Plasma 5.27.