Feature/websocket

Websocket implementation as provided from XSoftBG (with some minor adjustments from my side).

I'm aware that this is quite a structural change with possible impacts on other parts of the framework. However, for me it worked quite fine (even on mobile devices), and I did not notice any negative side effects. So I think it has the maturity to be tested in the field by other interested parties and in different scenarios (proxies, firewalls etc.).

Note: There was another feature related to focus managment which is somewhat related to server push(?) but I decided to handle that in another feature branch (code is commented out with //XXX remarks)