Add a lifecycle manager for xfconfd

As Tumbler does: https://gitlab.xfce.org/xfce/tumbler/-/blob/master/tumblerd/tumbler-lifecycle-manager.c.

Since xfconfd is D-Bus-activated, it would make sense that it terminates itself after a certain amount of downtime. Especially if it is used outside Xfce as a dependency of a single application, for example in a flatpak (see #27 (closed)), but not only.