Support/Implement FileTransfer portal
The FileTransfer portal is meant for c&p and DnD to work across sandboxed (and non-sandboxed) applications. Mutter here should implement the support for transforming URIs into the application/vnd.portal.files
target, so sandboxed drop targets are able to fetch the dropped files.
In the opposite direction, it should transform URIs from sandboxed applications into URIs that are readable on the outside.