scripted-streaming-chat.js

The goal of these changes is to reduce the "mode" specific javascript code in the chat implementation and reduce server-side guessing of browser capabilities.

- In the MessageRelay class from xotcl-core, we rework the javascript code sent to the connection when doing scripted streaming to make less assumptions about the functions defined in the parent scope

- In the chat code from xowiki, we move most of the logics to guess the mode into javascript, where we can test the actual capabilities of the browser without relying on the user agent

- The previously 4 javascript files dependant on the chat mode have been made into one