Unify aliased files in resource data

Qt resource files allow to add alias names to files.
For example in KDE we use that to emulate symlinks in our icon bundles.
This leads to multiple equal files being emitted to the resulting resource.
That could be avoided by omitting already seen files during emission and re-using the already written blob.
Perhaps I misunderstand how it works, but I tried to prepare a small patch.
If that is not the totally wrong track, I could try to submit it for review.