readspline: handle local file:// URLs

PCManFM passes file:// URLs 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
This result needs to be free with `g_free` AFAIK.