gtkfilechoosernativewin32: Fix support for non-ASCII paths

gtkfilechoosernativewin32: Fix support for non-ASCII paths

The code used SIGDN_URL to get an URL for the selected item, but Windows URLs
are a mix of unicode and percent encoded characters in the locale encoding
and not something GFile can understand. The result is a garbage file
path.

Instead use SIGDN_FILESYSPATH to get a real file path if available.

Also checks the return value of g_utf16_to_utf8 because file paths on
Windows can contain lone surrogates which would make the conversion fail.

https://bugzilla.gnome.org/show_bug.cgi?id=783347