[ENH] Adds class Engine linking the server with the state machine (wip)

Adds a Signal Slot implementation for the communication between the web
server and the state machine.
Any I/O using files (data, visible, actions) is bound to be replaced by 
transmission via websocket (wip)
Long-polling etc.. is now useless since all goes by websockets.
POST and GET could be replaced too.
[FIX] Adds viewer type to Player, and images to objects