tigefa4u projects Trac

Ported [2542] to 0.9-stable. Closes #2383.

Ported [2535:2536] to 0.9-stable. Closes #2369.

Correctly handle None values in ticket fields, especially when it involves dealing with Component owner. Closes #2383.

TracCrossReferences: always normalize paths before creating SubversionNode objects

This prevented cross-references to Source objects from working correctly:
sometimes the `id` had a "/" prefix, sometimes not.

TracCrossReferences: remove some leftover changes from TracDiff, hope the xref is clean now...

TracCrossReferences: the wiki syntax for creating relations is now provided by the !XRefModule.

Also, it is now made optional and has to be enabled by the following TracIni setting:
{{{
[xref]
semantic_wiki = true
}}}

TracCrossReferences: merge with log:trunk@...:2536

Also, removed some remaining bits from TracDiff.

Provide configurable MIME encoding support for e-mail notifications:
* Base64 (default)
* Quoted-Printable (recommended for western european language)
* No encoding (UTF-8 to ASCII translation)

Correctly interpret property changes occurring on the `repository_dir`. Should fix #2369.

Preparing the new Subversion test repository in order to be able to add unit tests for #2369