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
I'm not convinced by this change either. My first idea was also to combine flags and options. But because I wanted to avoid confusion regarding which state is the default as well as unpleasant statements like `m_property->option("enabled").value<bool>(true)`, I decided to add flags as a separate feature.