[SVG] Allow well-known IDs for SVG group elements

I generate class diagrams for Python code and would like to be able to split a class diagram into a set of linked class diagrams for large Python projects. While links out of SVG content are already possible, there exist no well-known targets in the SVG content.

The SVG code generated by PlantUML already includes unique entitiy IDs of the form ent0001 that can be used as SVG fragment identifiers in URLs. But those IDs are generated by PlantUML and not known at the time the PUML files are created.

In my use case, the absolute class names (class name prefixed by the names of the enclosing packages) are unique within a diagram and could function as entity IDs, if there was a way to specifiy that they should be used. In the general case, there should be a way to specify a unique ID for each entity (and an internal verification of its uniqueness).

It seems to me, this issue is related to #2080.