Implement `external-appstream-system-wide` key

Current implementation has gs_external_appstream_refresh_user as dead code and a memory leak. Actually use that function when external-appstream-system-wide=false and make sure the appstream plugin reads the files from where they were written.

I wonder, should the default change to external-appstream-system-wide=true? The code was actually working like that, and correctly implementing the default now means that some users might end up with redundant files installed.