Sources behind proxy #934

[falkoschindler]

This PR adds the windows.path_prefix to the src prop of ui.audio, ui.video, ui.interactive_image and ui.image.

How can we test it?

[rodja]

ui.video and ui.audio work as expected. But ui.image creates a js error with the missing prefix before loading correctly. And ui.interactive_image shows the js error compute_src is not defined.

Tested setup:

1. create a test.py with the demos
2. edit docker-compose.yml to use command: "./test.py"
3. start containers with ./docker.sh U
4. access http://localhost:8888/app

[falkoschindler]

Ok, there was an obvious bug in interactive_image.
But there are still requests causing a 404. It seems to affect both ui.image and ui.interactive_image. (Make sure to use different images and disable the cache to let the browser download both images if testing multiple UI elements at once.)

[falkoschindler]

I think I fixed it by setting the initial computed_src to undefined. This way the browser only sends a request when the source is actually computed.