[animation-triggers-1] formalize the <event-trigger-event> value

[ydaniv]

Currently the spec for values allowed as <event-trigger-event> is practically placeholder:

<event-trigger-event> = activate | interest | click | touch | dblclick | keypress(<string>) | ...

We should formalize the values for this type.

Options to consider:

1. Start with all events? Or start with a limited list and expand as we go?
2. Some events could benefit from a functional form with modifier args, as already implied in the spec, such as key for keyboard , and button for mouse/pointer, etc. Should we allow functional? Allow both functional and keyword? Only functional?
3. If we do allow functional form, which args for each event?
4. It's already suggested to add activate and interest (which IMO is great), but do we need activatein/activateout/interest/loseinterest? Or do we leave these to be automatically decided based on usage, i.e. event-trigger: --t interest / interest automatically becomes interest / loseinterest. Or perhaps interest(in|out)?

Proposal

1. Start with a limited list and expand. Probably after there's a first prototype deployed in the wild. I think starting with keyboard/mouse/pointer/interest/activate as suggested is a good start.
2. I think both are good (if it's not a problem).
3. I guess starting with the suggested key for keyboard events is fine as a starting point. Probably as a space delimited list? And if so, probably needs a special case for " " to be handled as Space.
4. I think starting with just interest/loseintereset, and activate being decided automatically based on usage should be a good start.

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cc @flackr @DavMila