avmedia: Implement "block untrusted referer links" feature

See f0a9ca24fd4bf79cac908bf0d6fdb8905dc504db "rhbz#887420 Implement 'block
untrusted referer links' feature" for details.  This adds some further /*TODO?*/
comments, and one known problem (marked /*TODO!*/) is that movies/sounds are not
blocked during a slideshow presentation.

Change-Id: Ib2d0c7e4f7b02c4bdec0d8a90cee5e7e1bee8325