change namespace to KSaneCore
and adjust naming of exported classes
since KSaneCore has not been released yet, no issues with API/ABI compatibility
Corresponding changes for Skanpage and libksane at https://invent.kde.org/astippich/skanpage/-/tree/adaptNamespace and https://invent.kde.org/astippich/libksane/-/tree/ksanecoreNamespace
(currently failing due to breaking API)