Namespacing LV2 ports

LV2 symbols named depends on the LV2 index but this cause symbols to change when we add a parameter, or where an optional port is there. Namespace them as in PR #516 because it is ugly.
Also: this is breaking, need an Option.

Review PR #516 first, then trace where LV2 symbols are generated from port indexes. Check how the proposed Option should preserve existing behavior and define the breaking-change boundary; done means symbols remain stable when parameters or optional ports change, with the new namespacing selectable.

Written by the indexing model from the issue text.