[LV2] LV2 port-group extension support in Ardour

[Robin Gareus]

Hi LV2ervs,

Ardour-git gained support for http://lv2plug.in/ns/ext/port-groups/ to
fan-out plugin-ports or multi-out plugins.

   https://vimeo.com/191024580

As per spec a plugin-port just need to define

   pg:group <SomePortGroup>
   lv2:designation <PortGroupElement>

The group specifies the relation between the designated elements and
also provides the Name. The element designation is just the count 0,..N

When fanned out, all elements in the same group end up on a single
target bus or track which is N channels wide.

The downside of this spec is that the port-groups are fixed in the .ttl
which isn't great for generic, general-purpose multi-channel synths.

Cheers!
robin