Add .Xidle

Add .Xidle

.Xidle is a script that will be called by xss-lock when started in
~/.dwmrc if it is present.  If this script is not present, xss-lock will
call i3lock directly.  If this script is present, it can do other stuff
before locking, and then more stuff after unlocking has completed.
Being used to suspend dunst messages and set XMPP status in mcabber.

i3lock execution has been changed to not use DPMS but instead a black
screen.  Hopefully this will solve some confusions where monitors that
are slow to sync don't provide enough feedback about what is going on.