refactor xml processor to enable global hooks available on all pages

[mikey]

The xml processor should be refactored in a way that it allows to register global hooks that add data to the dom tree on every page request.

There are two possible solutions:

1. Just add an empty method which may be overridden by extending classes. This has a good performance, but will lead to worse code in the xml processor.
2. Add possibility to configure a list of hook classes which are then called by the xml processor. Better code, but performance may suffer.

This is part of refactoring #129.

[mikey]

Implemented with r1414.