Add WM_CLASS atom

[tryone144]

As requested here this are the changes to the WM_CLASS atom.

The WM_CLASS is set to "instance", "class", where

element	description
class	programm name, Bar in this case
instance	programm instance, usually the executable name as passed by argv[0]

This should help write better wm rules for hiding the bar and stuff like that.

EDIT: Changed class to capitalized "Bar"

[tryone144]

WM_CLASS class name is now capitalized "Bar" in order to follow the convention to differentiate it from the instance name.