QSettings float should be human readable

Currently QSettings stores float values like name/float=@Variant(\0\0\0\x87@\xa0\0\0),
instead of name/float=100 which would be much easier to edit and uses less bytes.

It should switch to a human readable format, if the value is a valid float.
And this would be backward/forward compatible, because QSettings already reads the human-readable
numbers correctly.