tigefa4u projects Trac

TracDiff: unified diffs produced by Trac now correctly support added files

TracDiff:
* The ''Restricted Changeset'' view is now accessed through the `ChangesetModule`
* some minor cosmetic fixes

TracDiff: changed the Web interface for generalized diffs.

Now, the browser has a ''Diff ...'' button which leads to a page
containing a simple form with 4 text fields, enabling the
selection of the base path/rev and target path/rev.

TracDiff: access to the ''Restricted Changeset'' mode was broken

TracDiff:
* removed a few `print` statements
* clarified implementation notes for the `ArbitraryDiffModule`

TracDiff: refactored `ChangesetModule` and `DiffModule` to take advantage of [2086]: they now both inherit from the `AbstractDiffModule` class

TracDiff: merged with trunk (log:trunk#2039:2088)

Fix syntax error introduced in [2084].

Web-Admin: Mark `AbstractEnumAdminPage` as abstract.

* Make components inherit `implements()`, so that a subclass of a class implementing some extension point is also marked as extending that extension point.
* Allow marking component base classes as abstract.