Clear conversation window from a protocol prpl

[avictor.za at gmail.com]

hi Paul,

> However, unless anyone objects, I'm leaning toward changing the
> functionality of purple_conversation_clear_message_history() to trigger
> a clearing of the UI scrollback.

Fine with us.
The MXit prpl in im.pidgin.pidgin already calls
purple_conversation_clear_message_history().


Regards,
  Andrew Victor