Fix IbcEventType

[plafer]

The current assumption in our events is that each variant of IbcEvent has one and only one "event type" (which eventually makes it to the abci::Event::kind field). This is not true for IbcEvent::AppModule. For example, only in the token transfer app, it can have types ibc_transfer, fungible_token_packet, etc.

In order to fix this, we should

1. Remove IbcEventType (since not all event types are known by ibc-core)
2. let each event variant define its own event type