I tried to fix https://gitlab.com/OpenMW/openmw/-/issues/8080 by making it so that instead of crashing, we showed an error.
In doing so, I discovered some problems with plugin sorting and the refresh button, like:
* it forgetting the non-user content files somewhere
* nothing guaranteeing that built-in content files stay at the top of the list and them only being there because the first data directory that provides them is usually the first data directory
* it forgetting the non-user content files somewhere else
* it looking like it'd forget any kind of non-user setting under certain circumstances
I fixed those problems too
There's also handling for files declared as originating from a lower-priority openmw.cfg, e.g. anything in the local config or any intermediate ones, as they can't be disabled or reordered.
There's no way to mark such files yet, but the logic's the same as built-in files, so everything will be fine once that's set up.
