[Qt] coin-control features GUI cleanup 2 and 3

- 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
- 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 and rework the texts a little