As it was found that the only non-confusing way to port is to "create ONE new widget which contains a slider and a spinbox, and you pass the range it shall use", and the revision for KIntNumInput seems not be appropriate as well, this is a concept of a new solution.

https://phabricator.kde.org/D15223
https://phabricator.kde.org/D15316

Now it is just literally copy-paste of the current classes from kdelibs4support with all their glory (localization support and many unused features). Further work would be modernization and radical trimming down features to the level which is actually used by KolourPaint.

Is this an appropriate approach?

Compiles, installs. No differences should be observed when compared to the current state.