Use "filters" for the htmlParser API

[fredck]

Currently, we are introducing specific output manipulation directly into the domParser code. This makes that code quite hard to maintain, and less flexible, as it is not able to customize the manipulation with easy.

There should be instead a way to attach a "filter" to the parser, including all necessary rules for it.

In this the data processor can have a public filter property, which can be easily modified by a plugin or external code.

[fredck]

The provided patch introduces a quite drastic change in the way we manipulate data. It now parses it not only on output, but also on input (see #3003). Also, all the data transformation logic is now modular, so plugins can easily include their own filtering rules. The fakeobject system now uses it.

[fredck]

A good example of this code modularization is #3071.

[fredck]

Fixed with [3172].