[Qt] don't allow amount changes when AmountSpinBox is read-only
[Qt] don't allow amount changes when AmountSpinBox is read-only

- before it was possible to use the steps to change e.g. amouns of
  authenticated or unauthenticated payment requests (AmountSpinBox is
  already set to read-only here) - this is now fixed
- also move the reimplemented stepEnabled() function to the
  protected section of our class, where it belongs (see Qt doc)

wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin,wastholm/bitcoin