1.1.3dev: Wiki page version comments can now be edited. Refs #6573.

Requires WIKI_ADMIN permission.
Introduces `IWikiChangeListener.wiki_page_comment_modified()`.

git-svn-id: http://trac.edgewall.org/intertrac/log:/trunk@13198 af82e41b-90c4-0310-8c96-b1721e28e2e2