Because Plasma 5 themes need to be ported to work with Plasma 6, we need to show incompatible themes in the KCM separately, rather than intermixed, where users will try to use them and they won't work. If we don't do something like this, we'll get a *lot* of bug reports from frustrated users whose themes don't work and they can't figure out why.

Idea: a separate "incompatible themes" section in the grid view where they can live, with a little explanatory text saying that these themes need to be ported to Plasma 6 before they'll work.