form controls: validate *before* propagating value, but only on commit private/lmamane/validation

*Before* propagating, because after it is propagated it is too late!

Only on commit (unless the control is not commitable), because
validating each time the control value changes leads to a validator
being called each time the user enters a character of the value.
So each validator necessarily has to accept any prefix of a valid
value, which often defeats the purpose. E.g. one cannot do a valiator
that enforces a minimum length, or minimum password complexity.

So validating on commit makes much more sense. That's when the user
is trying to actually set the value as a complete value.

Change-Id: Id3142367bacd81e07d093c8bee97d518df28cafe

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