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
This is our standard pattern where each implementation includes its header file.
Seems strange but supports the following use cases without breaking any code:
 * Correct code highlighting and completion within IDES (at least QtCreator)
 * It's irrelevant if you include the .h or .hpp file, because it's not clear in some projects