Improve code comments on file path encodings

Try to clarify code comments where the string encoding of paths is unclear.

I changed mostly comments, except for a few places where the code was obviously wrong and the change could be made without changing other functions.

I don't know how to test the correctness of this change. On Windows and Linux, Inkscape uses UTF8 as native file path encoding (and I don't know about MacOS). On Linux, one can set G_FILENAME_ENCODING=ISO-8859-1 but this will crash even on basic operations like saving to a file named ä.svg.

I still hope the change makes it easier to migrate to Gio::File one day.

Feel free to discuss if this merge request is actually helpful - it is a byproduct of my work on the Export dialog. I can also understand if you think we should give up any such "reverse-engineering" and rebuild everything with Gio::File.