tigefa4u projects Trac

Follow-up to r2818: now the `rss_title` function should do what I wanted.

Trac RSS 2.0 feeds should now be valid.

The RSS feeds produced by the TracTimeline, the TracQuery, the TracReport
and the TracRevisionLog should now work as expected.

The following changes have been implemented:
* The item's <title> element should never contain HTML markup,
and only numerical or predefined XML entities are allowed.
* The item's <description> element contains ''escaped'' HTML markup
* Also, the TracRevisionLog RSS feed's <author> elements are not
emitted if there's no author (i.e. no e-mail address for the author)

Closes #2580.

The Changeset timeline event has been improved:
* The RSS title doesn't contain markup anymore
* The readability of the included file listing has been improved
(closes #2072).

TracCrossReferences: bug fix for Ticket timeline bug introduced in r2788

The refactoring of TracCrossReferences continues...

The rebuilding of the cross-references is now done in the `xref.module`.

Consequently, a `ITracObjectManager` has now less to do:
It only has to provide the db type it manages (only one),
the factory method to create the subclass of `TracObject`
that it manages and an iterator over all existing objects
of that kind.

At the `TracObject` level, a bit more generic functionality
concerning the DB interface has been added:
* a `_load` method that can be used to ensure a `TracObject`
is in it's loaded form. `reload` should still be used if
one wants to ''force'' reading from the DB.

The facets are now better handled at the level of each `TracObject`
(there will be soon a `WikiFacet` class again, I guess).

''View changes'' should trigger a `GET`, not a `POST`. Thanks to mgood for the tip.

Title for generalized diff is a bit more compact: ''Changes from `_old_path_` at r`xy` to `_new_path_` at r`yz`''

Only show the ''View changes'' button if there are more than 10 entries shown. This change concerns both the revision log and the wiki history.

TracDiff cleanup: the ''View changes'' button on TracRevisionLog now directly sends the diff request to the TracChangeset module (no redirection involved anymore)

TracDiff: the title for displaying a set of changes on a given path is now more compact, and links to the corresponding TracLogRevision view