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

[johguenther]

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