readspline: handle local file:// URLs

PCManFM passes file:// URIs as parameters when opening directly from the explorer. Decode the URL and treat it as a local file.

[Diff without whitespace](https://github.com/fontforge/fontforge/pull/1494/files?w=1)

Closes #1452.

r? @frank-trampe
Okay apparently it does not get reset at line 986/990 because that's only if it matches compressed file extensions at line 970/975. I think that replacing here is the right thing to do.