Rework message handling.

Rework message handling.

- Move all message-related types to their own header to make moving
  them to a cross-module events easier.
- Rename OnUserMessage to OnUserPostMessage.
- Rename OnText to OnUserMessage.
- Replace the dest, target_type, and status parameters with the
  MessageTarget class.
- Replace the text, exempt_list, and msgtype parameters with the
  MessageDetails struct.
- Add echooriginal and originaltext to the MessageDetails struct
  to allow spam filtering to not be broken by cap echo-message.