Draft: Addition of an 'internal pad' to every element of the pipeline.

Introduces new a InternalPad as part of the ElementLike base class. Every element is initialized with one InternalPad attribute. Its position in the graph is between all sink and source pads of its element (all sink pads point to the internal pad, which points to all source pads). The corresponding function call does nothing by default, but can be overridden in an Element class in order to do some task after an element's sink pads are called and before its source pads are called.