Reflectionzeug refinements

* Restore default behavior for property guis
* Remove flags in favor of a shared usage of options
* Add convenience accessors for property options
* Fix include style
* Remove obsolete member variables and corresponding functions from Enum- and StringProperty
* Add toVariant to PropertyGroup
The properties aren't the only use case. It's also used by scriptzeug. The advantage of constructors is that they are faster, because the object isn't first initialized with default values. Anyway, it's the same approach as implemented by `QVariant`.