kcm: Factor out ComboBoxWithCustomValue for later reuse
CC @merritt whose commit c82ad686 I mangled to the point of unrecognizability. But it's still derived work, so I need your OK for relicensing the file to LGPL here as well.
Turned out tricky, but I'm pretty happy with the API for the most part. Hopefully reviewers will agree, but if not then help me make it better!
Cuts down on a lot of boilerplate for the code that's using the component. Likely also a better separation of concerns. Model types remain hardcoded, but for most use cases this won't pose a problem.
Hoping to submit the component to Kirigami or another library of common components, to make use of it in other KCMs as well.
Edited by Jakob Petsovits