examples/widgets/spinboxes: a spinbox containing a number prepended by a '+' doesn't accept to append a new number

1. try to enter 20 in the topleft spinbox. (if you type the '' and the
spinbox is empty this is not possible, so you have to add a number
first, and then prepend the '+')
2. Now place the cursor at the end and try to delete the '20' (e.g. you
want to type another number). This is not possible, and very annoying
until you understand why.
With the '-' symbol everything works as expected.

Note: The validator lineedits in examples\widgets\lineedits has the same
problem so this indicates that it is a validator thing.
its also the same problem with the doublevalidator.....

This is not a regression from 4.1.1