Background handling for HLS videos/streams

 * Always set background color for `is-playing` and remove splash image
 * End streams 0.4s earlier and hide video tag on finish

Background-color for `is-playing` affects also normal VOD streams and
cannot be disabled with other means than overriding css.

End stream fix and video tag hiding can be disabled with
`flowplayer.conf.hlsFix = false`

refs #479