Experiment: make ref wrappers optional

[dpwiz]

Replace mutable ref + changed flag for monadic action yielding the change.

StateRef behaviour is trivially recovered with a maybe-to-set/bool wrapper and a bunch of binds.

[dpwiz]

The wrapped functions aren't even needed if widgets are to have their "mutable" arguments stated last.

There can be statefulN-family for widgets that mutate N arguments at once.

-- With all arguments expanded
dragFloatRange2 desc speed minValue maxValue minFmt maxFmt refMin refMax = ...

-- Just add your values
dragFloatRange01 label = dragFloatRange2 label 0.01 0 1 "%.1f" "%.1f"

action rMin rMax = do
  changed <- stateful2 rMin rMax $
    dragFloatRange01 "from zero to hero"
  when changed do
    ...