It still doesn't work for converted ugoira animations thanks to how
those files are handled, but everything else, including files with
unknown or changing file extension, now works as it should.
- change 'has_extension' from a simple flag/bool to a field that
contains the original filename extension
- rename 'keywords' to 'kwdict' and some other stuff as well
- inline 'adjust_path()'
- put enumeration index before filename extension (#306)
