Improve alignment annotation data model

The jalview.datamodel.AlignmentAnnotation object has become overloaded with the addition of new features such as HMM profiles, PAE matrices, and alternative forms of consensus rows, and increasing availability of per-sequence annotation tracks from disorder, structure prediction, and local structure descriptors (dssp, stride, and 3di).

The time has come to formalise the way in which relationship between source and an alignment annotation row are represented.

Requirements are:
- efficient (JAL-1463)
- easy to understand - particularly for new developers
--> clear documentation
- allows import of legacy projects/annotation tracks