Free GL eventhandlers

When user deletes a WebGL context, disconnect all event handlers associated with that context so that phantom GL context loss events or similar are not passed to user from contexts that are already supposed to be freed.
That's a good point - no, `JSEvents` might not actually be there. I can't think of any other way than to do a `if (JSEvents) JSEvents.removeAllHandlersOnTarget(...)` to make this gracefully ignore if `JSEvents` had not been used in the first place. How does that sound?