tigefa4u projects Trac

Property renderer branch: a few bugfixes that I forgot to integrate properly in the other changesets
- more informative display of svn:externals properties for which no mapping is defined
- fix display of deleted properties in changeset view

Property renderer branch: add a `SubversionPropertyRenderer` which knows how to render the `svn:externals` property (#2340).

This renderer reads a `[svn:externals]` section in the TracIni, which looks like this:
{{{
[svn:externals]
http://ourserver.ourdomain.com/svn/eng-soft http://localhost:8000/devel/browser/tags/$path?rev=$rev
svn://dserver/support_repository http://dserver/trac/support/source/$path?rev=$rev
}}}

That way, the configured `svn:externals` entries will link to the right place in other Trac (but the URL can point to other servers, like ViewVC or directly in the Subversion server through the http: protocol).

Property renderer branch: add IPropertyRenderer and IPropertyDiffRenderer interfaces

This also provides default implementation for those interfaces, which reproduce the current behavior, except for the default `DefaultPropertyDiffRenderer` which implements #3002 (view difference of multi-line properties as rendered diffs).

In order to implement the above, a few changes in the `PatchRenderer` were needed.

Also, the `.revision` property of the WikiContext which was added in the blame branch was also needed here, so I ported that part of r4483.

The main ticket for this is #1601.

Start a work branch to implement IPropertyRenderer (#1601) and fix related tickets

add import of `Context` missing from r4509 (fixes #4506)

Blame branch: better handle failure of an annotator

Instead of retrying all preview renderers and failing again for each of them, an error with an annotator (assuming it happens at the `get_annotation_data` stage) will now simply omit the result of this annotation from the output, leaving the error message accessible in the title info.

In the case of the blame annotator, such error typically correspond to SVN thinking it can't get the annotations because the file is binary. With `client.blame3`, it would be possible to override this check, but only `client.blame2` is currently usable in the bindings as of Subversion 1.4.2.

Blame branch: minor tweak to the changeset panel show and resize code.

Blame branch: sync with trunk (merged [4501-4502,4509-4510,4517])

Changeset UI fix for empty messages

Blame branch: small cleanup in the Javascript