Also suspend screensaver when idle watcher is enabled

[tsujan]

Without this, the screensaver timeout will kick in and interfere with our daemon. The suspension will be reverted when the idle watcher is disabled or the daemon is stopped.

Fixes https://github.com/lxqt/lxqt-powermanagement/issues/374 completely

[tsujan]

@stefonarch If you use a screen-locker, could you please check whether it works with this patch (when "Lock Screen" is chosen in the idle watcher)? It should work, but I've seen too may unexpected things about DPMS.

[stefonarch]

It works fine here I see, using xautolock and i3lock xautolock -time 20 -locker i3lock-fancy

[tsujan]

Any objection to merging?