Decimal, Double and Float validators don't allow a leading "+" sign

Running the TextInputValidatorTest program with some edge cases revealed that although the validators allow a leading "-" for numbers (as they should), a leading "+" sign gives a validation error or "not valid" status, which should be perfectly acceptable.

The underlying problem seems to be that since the DecimalFormat normally used doesn't really work right if you specify a leading positive string (that is, it will require it, not just allow it), that things are a bit wonky there.  So, we will have to allow it ourselves with code outside the "Format" validation.