org.filezillaproject.Filezilla is wrong; use org.filezilla_project.FileZilla instead

Per https://github.com/ximion/appstream/issues/162#issuecomment-355335480:

> > One such example is FileZilla, which was assigned org.filezillaproject.Filezilla. I gather this is incorrect because filezillaproject != filezilla-project and should have been filezilla_project?
>
> Yes, Flathub was wrong to apply that transformation. Nobody can own filezilla_project.org because underscores aren't allowed in domain names, so it can't possibly collide with a real domain name, which is why normalizing/escaping filezilla-project to filezilla_project is an OK thing to do. (Similarly, nobody can own _7_zip.org because domain name components can't start with underscores, so it's OK to escape 7-zip as _7_zip, which is the example I like to use in places like the D-Bus Specification.)

Flathub was wrong to use the AppStream ID `org.filezillaproject.Filezilla` for filezilla because their actual domain name is filezilla-project.org, and the hyphen should have been turned into an underscore, not removed.

Also, the last part of the string should be `FileZilla`, not `Filezilla`.

We should do this once we can rename AppStream IDs on Flathub (https://github.com/flathub/flathub/issues/227).