VerifyChecksumsController: Decode filenames from Utf8

On windows, QFile::[en,de]codeFileName convertes from/to an ANSI (8 bit) encoding (see QFile docs).
This breaks when filenames contain unicode, where non-ANSI characters get converted to '?'.

This patch (in combination with the accompanying new tool in gnupg) thus fixes checksum verification
for filenames containing unicode characters.

• GnuPG-bug-id: T6574