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"

[ThomasAdam]

@tryone144 -- if you're going to set the class name to "bar", give it a capital letter, "Bar". This is the convention to differentiate it absolute from the instance name: "foo, Foo" for example.

[tryone144]

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