Clear sent message history when clearing chat view

When user clicks Action->Clear View, clear the sent message history
in ChatTextEdit too - when you want to delete a sensitive conversation,
keeping sent messages is not a good thing to do :)

Also fix the actual Clear View action by setting d->exchangeMessageCount
to 0 in clear(), otherwise last X messages from backlog are loaded after
re-initialization of the chat view.

REVIEW: 113255