mosaic: cleanup option handling using range enforcement

 - cleaner code.
 - places helpful bounds on controls in gui prefs that prevent cycling out
   of range.
 - results in range hints being given in help output.

note that it is correct for ranges to be added where there is also a
choice list. ranges are enforced, whilst choice lists are not. with a
choice lists you're restricted to a droplist in gui prefs, however any
value can come from CLI or saved settings. a range is enforced upon storing
the value where ever it came from. thus it is correct and useful for them
to be used together.