Add a background radio player

This adds a new background radio player that currently can be used by a theme to play a radio stream as background music or the web cam viewer can have radio feeds associated with a web cam feed. For example many of the Virtual Railfan web cams have associated radio scanner audio streams which have the dialog between the dispatchers and train drivers or other rail maintenance workers etc.

A screenshot of the webcam viewer screen with the Radio Player dialog overlayed.