Propertyguizeug cleanup

things done:
* revised all classes regarding whitespace and minor style
* revised all editors
* revert implicit variant interface of `EnumProperty` because
  * variants are not meant to be used like that
  * explicit is faster and less error prone (compile time checks)
* introduce `PropertyEditorPlugin` and `PropertyPainterPlugin` and subsequently
  * move editor paint code to editor classes
  * adapt `PropertyBrowser` and `PropertyDelegate` 
  * move affixes code (number properties) to editors