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.

On other platforms, this is effectively no change, since the conversion was already done from/to utf8.