Explicit event representation

In a similar vein as #2603, I'd like to make all events explicit in the XML.

We implicitly generate events of the form "{venue}-{year}" for every venue referenced in a volume. For example, the following XML ...

```xml

...
2014
gwc
```

... induces an event ["gwc-2014"](https://aclanthology.org/events/gwc-2014/).

However, old-style collections like W14 contain many different workshops, and therefore many different "events". This creates some challenges to making these events explicit:

1. The schema currently only allows a single `` per collection.
2. An explicitly defined `` lists `` volume IDs. This doesn't include the volume IDs defined in the same XML file, as they are assumed to be part of the event automatically.
3. Thus, if we allowed multiple `` definitions per collection, we'd have to list _all_ associated volume IDs explicitly. This is because an event like "gwc-2014" should only include "W14-1", but not all the other volumes defined in W14.

In my opinion, this raises the question if events should be defined in the collection XML at all. In the library, if we want to find an event with ID "gwc-2014", it is undeterminable which file will contain this event (_except for the rather loose association that it's probably in a "?14.xml" or "2014.*.xml" file_). On the other hand, events can also define bibliographic items like talks, which we also hope to expand in the future — and those clearly belong in the XML.

I'm not sure how best to resolve this yet, and welcome opinions.