Add style classes

[Zoxc]

This adds style classes. They can be declared with the style_class! macro:

style_class!(pub Button);

There is a view decorator to apply them:

view.class(Button)

And a method on Style to apply styles only to a class:

s.class(Button, |s| s.border(1.0))

The scroll view now uses classes for the handle and track styles. The window scale example is updated to use a button class.

[dzhou121]

This is really cool!