Another reason: File system and registry names depend on it

An experimental change sneaked in.

its only caller is the singletons instance() method

- more sizes
- rename state icons due to a cmake limitation
  (it globs owncloud-icon-* for mac and windows icons,
   which causes the state icons to appear as app icons)
- adjust resourcefile and theme accordingly

Also remove bogus manual line break.

This turns the Theme class into a singleton.

The base class cannot reasonably assume anything about
pretty printing. The subclasses know a lot better.