xsession: don't rfkill/xbacklight

Recent versions of systemd handle that without anything special.
lock: use xss-lock instead of xautolock

xss-lock features nice integration with standard X screensaver
feature which makes it easy to trigger it or to disable it. Moreover, it
also has some logind integration to lock screen before suspend.
lock: sleep a bit to let i3lock lock the screen

We could let i3lock fork to notify us but then, we have to spy on it to
know when the screen is unlocked.
lock: start xss-lock in xsession

It seems that spawning it will just crash awesome.
xsession: reenable rfkill

Not handled by systemd. However, backlight is kept between reboots.
lock: fade screen during 10 seconds
lock: use more steps to fade screen
lock: ensure notification happens before timeout