Update styles to support sub-styles
Styles like occupied and focus are logically just sub-styles of the :views style and should therefore use a special syntax. Also the :urgent style should be available for view only.
Additionally the sub-styles can be used for styling of sublets independently as sub-styles of the :sublets style and replace the foreground, background, text_fg and icon_fg config values. When icon_fg is gone another style value for icons is requiered.
Appropriate Subtlext methods (#style=) to set styles could replace the
#background= methods of