[Qt] coin control change address handling update

- re-work change address handling so that default is CNoDestination(),
  until a verified and known change address was entered (easier code flow)
- add a missing NULL pointer check for adresstablemodel
- add a missing text when opening coin control address selection for
  priority and ensure the label is black
- add a missing . at the end of a sentence
Merge pull request #3391

3380713 [Qt] coin control change address handling update (Philip Kaufmann)