server: dynamic pages first draft: /!\ new dependency: autobahn This patch introduce server part of dynamic pages. Dynamic pages use websockets to establish constant connection with a Libervia page, allowing to receive real time data or update it. The feature is activated by specifying "dynamic = true" in the page. Once activated, page can implement "on_data" method which will be called when data are sent by the page. To send data the other way, the page can use request.sendData. The new "registerSignal" method allows to use an "on_signal" method to be called each time given signal is received, with automatic (and optional) filtering on profile. New renderPartial and renderAndUpdate method allow to append new HTML elements to the dynamic page.