Adds iframe windowing to demo server

[sohkai]

Enables up to four separate windows to be shown when using the demo server.

Things to note:

• History can be a little bit wonky, mostly when you try to go back on a frame that has no more back history
• React devtool can only be attached to one frame at a time, so debugging might be more difficult when using multiple frames.
• React devtool has a bit of a hard time attaching and reattaching to all of these frames; in particular, it doesn't really know how to clean up after itself so successive reattachments will be a new app in it.

[diminator]

Looks all good The devtools were a bit wonky indeed, but those will only be used in the specific app (by routing to the url in fullscreen)