documentation: update text to match the last modification
admin: don't compute background offset if the display is disconnected
android: provide a way for the receiver to safely reload

When using `window.location.reload()`, we have the risk to be unable to
reload correctly due to network problems. We request the application to
reload for us instead (if possible).
receiver: factor supervisor code in its own module

This is the code to handle reload/readiness with the container.
receiver: on uncaught error, ask for a reload