Handling locking and other screensaver related things in power manager

[Jarno Suni]

Currently there is a complicated code deciding how to lock in function
xfce_screensaver_lock in xfce4-power-manager/src/xfce-screensaver.c
I think it could be replaced simply by calling xflock4 sychronously.
I wonder if the power manager has to know about running screensaver
daemon then? There are functions like xfce_screensaver_inhibit that
use information about screensaver type, but could that be done by
xdg-screensaver commands suspend, resume and status (support of which
should be implemented for xfce4-screensaver)?

I have recently been working with adding support for
xfce4-screensaver:
https://gitlab.freedesktop.org/xdg/xdg-utils/-/merge_requests/38