Generally fix MessageProcessor to match the original design

- Make Message constructor private and a friend class of the message processor. This simplifies code, and makes accidentally skipping the escape filtering impossible
- Message now has two extra properties isHistory and messageDirection
- Message filters have access to the "message context" the account/channel in which the message was received
- Remove the broken filterIncomingMessage / filterOutgoingMessages with just one method. Plugins can use messageDirection to get the same functionality

This is a setup to fix a range of bugs in the message filters