xr-iframe deletion ability

[chrislatorres]

Perhaps we should allow for deleting xr-iframes that have been opened. The reason why this might not be worth doing is because this is just to serve as a demo and not an actual way of managing and experiencing reality tabs.

[avaer]

I think it’s fine to have select and delete in this index.html.

Actually that is probably a good thing to have available for testing, aside from any end user browser.

[avaer]

Added in index.html.