config: allow `snap-name`, `surface-title` to be a list

[Saviq]

Now with #191 it makes sense for multiple clients on an output.

We should add support for comma-separated lists for snap-name and surface-title.

[AlanGriffiths]

Well, it's yaml, so an array would be better than parsing a value ourselves.

But the value of this is going to be limited without:

1. multi-seat
2. a per-monitor task switcher

[Saviq]

Well, it's yaml, so an array would be better than parsing a value ourselves.

Wanted to reduce what we need to pass through, but sure.

1. multi-seat

Don't think that applies — that's a session + compositor per seat.

2. a per-monitor task switcher

Yes, you could only switch on the one monitor until then.