add test for appimages in debian packages

Marc Leemanrequested to merge
erpel/lintian:feature/test-for-appimages into master

This test was added because appimages are in essence a black box that is pulled into the OS, making the licences, build reproduceability opaque. It also introduces security issues.

Therefore, it is best to avoid these and warn about it.

Merge request reports