Add apparmor rule to fix thumbnails (noble SRU)

Desktop Icons NG uses libgnome-shell to generate thumbnails. Internally, it uses bwrap to execute the thumbnailer inside a container. Unfortunately, this no loger works directly due to the security changes done in Ubuntu 23.10, which disables, by default, user namespaces, and requires specific rule files for each application that needs it.

More info in https://ubuntu.com/blog/ubuntu-23-10-restricted-unprivileged-user-namespaces

This patch adds an apparmor exception file to ensure that it works. Without it, the thumbnailer just fails.

It is paramount to merge this merge request AFTER !7 (merged), to keep right the history in GIT.