alsa-ucm: Split out device group helpers from combination port code

The things I want to do for multiple profiles per verb largely overlay with what combination ports already do, so I split them out into helper functions working on idxsets. I also made ports store their devices as idxsets for uniformity.

The conformance check was slightly wrong and was preventing combined ports from being created, this also fixes that. Now devices which share a PCM and are not marked as conflicting devices can generate combination ports, but those marked as conflicting still do not.

(This contains commits from !587 (closed) and !588 (closed), you might want to handle those first. Should be easier to read commit-by-commit as well.)