0006515: Error building narrated web

Circular references (either direct or indirect) involving media->citation->media or media->citation->source->media cause NarrativeWeb to crash with "maximum recursion depth exceeded". Fix NarrativeWeb when constructing list of objects to be output by checking whether a media object has already been encountered.

Also fixes
0006418: Navweb crashes alpha4
0006630: Export Erzählende Webseite
0006643: Crash during making webstek report
0006652: [NarrativeWeb] Circular References lead to endless recursion (crash)
0006654: Unable to created narrated web report

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/maintenance/gramps40@22269 4ae1f11a-8b86-4847-b8af-ab372f36d1fd