Make the WebGL not supported screen configurable

[ksons]

Currently a red screen appears when WebGL is not supported. Make this configurable.

[lachsen]

I added a new 'unsupported' event in the current event branch.
Before the default error message is printed, the user receives this event.
When calling event.preventDefault() during the callback, no error message will be printed. That way, the user can create a custom error message.

So this should solve this issue (once we merge it into develop)

[ksons]

Fixed in 42509fe

[ksons]

See also: https://github.com/xml3d/xml3d.js/wiki/Events#unsupported