[dev] [en] rich text in sendMessage

[Goffi]

G'day,

I have added the handling of sendMessage in XEP-0071 plugin, so it's now 
possible to send rich chat text, throught it's not yet managed in frontends.

To send a rich text, the "extra" (formely "options") dictionary of 
sendMessage must have a "rich" element with the rich text in the 
currently selected syntax.

sendMessage now use a deferred which can be used by plugins to modify 
the behaviour. sendMessageXml trigger has been removed as it is now 
obsolete.

I have also changed the history management: if "message" data (i.e.: 
text only version of the message) is empty, the message is not saved to 
database. It's avoir the storage of empty messages (that was happening 
with the chat state plugin).

++
Goffi