Add a "clean" CSS class for untouched fields

[lionel-bijaoui]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Feature

• What is the current behavior? (You can also link to an open issue here)
  Validator: Field validated class #253

• What is the new behavior (if this is a feature change)?

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  Field don't have a valid class at start. They start with clean and once they are edited or validated they loose it. Then, they either have valid or error class.

• Other information:
  • Add "clean" when the value is not manually changed or validated
  • If changed or validated, loose the clean state and is either 'valid" or "error"
  • Class name can be customised with "validationCleanClass"
  • use internal "vfgTouched" value to track state of field

[coveralls]

Coverage increased (+0.2%) to 87.689% when pulling 4f6b05e on lionel-bijaoui:lb_clean_validation_state into b02d866 on vue-generators:v3.