Gwt canvas

Changes for the GWT client:
- Under the hood: completely refactoring of the canvas drawing mechanism.
- Management of environment as layers. Each environment (with its rooms and objects) belongs to the same layer.
- The base classes are prepared to create extend the use of layers by the user. The graphical part still need some work.
- First test of "double-click" to zoom. If the user doubleclicks over a room it will zoom the room.