setDoubleStep works in the same manner as for setIntStep in that it is
just available for input dialogs getting a double.
[ChangeLog][QtWidgets][QInputDialog] Added setDoubleStep to enable
changing of the step amount for getDouble().
Task-number: QTBUG-17547
Change-Id: I5cabcfceb23324f8045f2b1e49017644418db01a
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>