[Qt] coin-control features GUI cleanup 2

- remove monospace labels from sendcoinsdialog also
- use a validated line edit for the change address
- add a tooltip to change address switch
- ensure we have a valid change address in
  CoinControlDialog::coinControl->destChange or just CNoDestination()
- some small ui file changes
[Qt] coin-control features GUI cleanup 3

- remove style sheets from ui files and use Qt attributes instead
- make some more strings untranslatable, to make life for translators
  easier
- split up long tooltips an rework the texts a little
Merge pull request #3285

d3207b6 [Qt] coin-control features GUI cleanup 3 (Philip Kaufmann)
834e14e [Qt] coin-control features GUI cleanup 2 (Philip Kaufmann)