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
