add user-definable 'canUseUMapFunc' to UMapDef

so that we can tie certain UMaps very specifically to certain specs or
value types
add 'delay' arg to line UMapDef

this can makes the line wait at the start point for a while. Please
note that the 'range' can also be an UMap, so line can be used to
auto-crossfade between UMaps.
introduce 'utility' category for UMapDefs
use canUseUMapFunc for rho_theta UMapDef
add 'crossfade' UMapDef

to be able to crossfade between values or UMaps