msglist: Support outbox messages in message list

This adds some overhead in magnitude of O(1) (where the constant is
the number of outbox messages in a view, expected to be small) on
message event handling.

We add outboxMessages as a list independent from messages on
_MessageSequence.  Because outbox messages are not rendered
(the raw content is shown as plain text), we leave the 1-1
relationship between `messages` and `contents` unchanged.

When computing `items`, we now start to look at `outboxMessages` as
well, with the guarantee that the items related to outbox messages
always come after those for other messages.  Look for places that call
`_processOutboxMessage(int index)` for references, and the changes to
`checkInvariants` on how this affects the message list invariants.

`addOutboxMessage` is similar to `handleMessage`.  However, outbox
messages do not rely on the fetched state, i.e. they can be
synchronously updated when the message list view was first
initialized.

Author: PIG208

lib/model/message.dart

@@ -542,6 +542,8 @@ class MessageStoreImpl extends PerAccountStoreBase with MessageStore, _OutboxMes
       }
     }
 
+    // TODO predict outbox message moves using propagateMode
+
     for (final view in _messageListViews) {
       view.messagesMoved(messageMove: messageMove, messageIds: event.messageIds);
     }