Bugfixes with handling of visibilty.
Fixed getBoundingBox() methode of <model> element.
renderer.CanvasHandler.canvasSizeChanged(): added propagation of resize event

Now we can register a listener on the xml3d element for "resize". The event's
detail will hold an object with properties width and height.
interface-elements: removed a wrong return statement
Critical Xflow fix.
Implemented Xflow operator overloading.

There can now be several operators with the same name.
Operators with matching input types is choosen.
If no operator matches, a detailed list of all operators