Implement Internal Player Seekability for Proxied Playback

[oakesville]

Proxying is needed because libvlc does not support HTTP Basic or Digest authentication. At the moment MediaStreamProxy does not attempt to support seekability in the proxied stream. Generally this means external playback using the built-in player does not support seek function:
public boolean isItemSeekable() {
return itemLength > 0 && proxy == null;
}
This enhancement will require an understanding of the libvlc streaming seek behavior.