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
Imho the problem is that you usually don't want three states but two. And in these cases the third state is redundant. In addition you always have to document what this third state represents: `true` or `false`. It's pretty clear for `"enabled"` but for flags like `"isfile"` it's not which forces users of the library repeatedly to look at the documentation. Not very convenient. A possible solution would be to store a `void` variant instead of a `bool` one. This doesn't solve the negative logic issue, though.