GUI for custom screenlocker on x11

[stefonarch]

Needs https://github.com/lxqt/liblxqt/pull/346

Settings go to session.conf which is more natural as also the delay is here and it is about session locking. The PR above checks for backward compatibility first lxqt.conf.

[General]
__userfile__=true
leave_confirmation=false
lock_command=slock
lock_screen_before_power_actions=true
power_actions_after_lock_delay=2250
window_manager=kwin_x11

We could also remove the "custom" and use xdg-screensaver lock as empty/default value.

In the list I added all screenlockers from here, except AUR.

Window Manager → X11 Window Manager

[stefonarch]

With some help from @tsujan done.

[luis-pereira]

Testing it

[stefonarch]

@luis-pereira Updates?