[Azoth] Added UnreadQueueManager::messagesCleared() signal.

For #1445.
[Azoth] Delegated managing unread count to UnreadQueueManager.

For #1445.
[Azoth] IProxyObject::MarkMessagesAsRead() for #1445.
[Azoth Xoox] Removed messagesAreRead() signal.

It's not used anywhere anyway.
[Azoth Xoox] Truly mark messages as read on forwarding.

This probably closes #1445.