Draft: mobileshellsettings: refactor to make accessible from C++

To be able to fix #408 we need to be able to access our shellsettings from C++ to be able to register and deregister the gesture when the setting is toggled.

Problem is: it's utterly broken and it has utterly broken me. My CMake-fu seems to not be good enough for this.

I moved over to using KConfigXT and it all seems to compile and be able to be linked to other parts of plasma mobile and it even seems to work in most places like editing settings in the mobileshell kcm and having it apply (eg: disabling/enabling the navigation bar) - buuuuut it's also absolutely not working when it's trying to query settings in Folio and I have no clue why. It's probably super obvious, but I need to pause this for a bit and come back with a fresh mind.

If this gets done we may want to cherry-pick this. It's a bit of a big change for cherry pick, but also kind of important, not sure.