xcf: Fix regression when reading files with "unsupported" properties

Summary:
The fact that we don't know the property is most of the times not fatal,
so what we have to do is just "skip" the property and hope for the best

BUGS: 411327

Reviewers: cfeck, apol, vkrause

Reviewed By: vkrause

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D24114