Fix broken FileZilla package: change URL

Your FileZilla package is not updating anymore, because the installer (starting from 3.9.0) is now only downloadable via SourceForge. As you probably know, SourceForge is a bit tricky to use with Ketarin, because Ketarin lacks of following HTTP 301/302 redirects which are used there.

I had to “misuse” the `{{DownloadUrlx64}}` substitution variable as 32-bit download URL, otherwise the package would contain a specific mirror URL, which is bad (not all mirrors aren’t always online and some are slow sometimes).

No worries about the adware that was bundled with the FileZilla installer on Windows some time ago. The installer is now adware-free.
Is this sourceforge project dependent? I can't seem to get the exact same thing to work on stexbar. Not sure what might be different, but it does not bring up the download url