Add msg port support to GRC de/serialization

[frankosterfeld]

When I add custom msgIn/out ports and connect them, I get ports with subindex in the connections, like [1, 0], ..., [1, 1]. This seems wrong and fails on load, as msg ports have no subindexes.
I would suggest we follow the old grc format from GR3 and support port names in the connections section, like

 - [foo, '0',  bar, '5'] # streaming ports referenced by index
 - [foo, ['0', '4'], bar, ['1', '3']] # streaming port collections (should the subindex be quoted??)
 - [foo, [out, '4'], bar, [in, '3']] # streaming port collections using names
 - [foo, msgOut, bar, msgIn ] # connecting msg ports, by name

That way streaming and msg port indexes wouldn't be mixed up, and the yaml would be easier to read, too.