Better support for GUI building, e.g. soft-min/max info of parameters

this was feedback from the hackathon
- ~~when only slider widgets are available, then the absolute `minimum` / `maximum` is of little help and not useful (e.g. for unbounded values like `spp`); a typical/useful range would help~~
- how to deal with scene-dependence? e.g. meaningful `attenuationDistance` or AODistance depend on the scale of the scene, but parameter information are compile-time static
- hints for widget selection