[js] add 'pointer' class for objects such as network interfaces and canvases
[net] use 'pointer' object in netif js binding instead of object with netif id
[graphics] do not use canvas_id when fetching canvases from canvas objects
[graphics] use pointer object for canvas
[graphics] use common function for fetching canvas from graphics object
[graphics] save canvas pointer in graphics object
[graphics] no more need for Scanvas_id
[graphucs] no need for canvas_register anymore