Selection between SoftwareCenter app and PowershellApp

[Hattshire]

Why not having a single string based setting to select which thing to use instead of too redundant booleans?

Also a fallback option on the setting in my opinion would be great instead of choosing only one of those and failing on systems without ConfigMgr client when the preferred thing is SoftwareCenter.

I would do a pull request myself if needed, but I don't know if the purpose of using booleans instead is due to some issue or just a design decision.