Add support for kde-inhibit

[fohrloop]

This would be a non-dbus alternative on KDE. This together with #404 might make the jeepney python package dependency optional #65)

• See: https://invent.kde.org/plasma/kde-cli-tools/-/merge_requests/2

Since the MR has Milestone "5.20", I assume this has been available since KDE Plasma 5.20 (13 Oct 2020).