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
In this case it is possible to forbid the implicit constructor of all Variant types and use solely the static one.
With such a convenient interface on the properties this wouldn't matter.