Remove unused addTextTracks method
Add remoteTextTracks and removeRemoteTextTracks.
removeTextTracks mimics textTracks but only keeps track of the remote
text text tracks, i.e., the text track elements or tracks we downloaded
ourselves.
Rework displaying of captions
Listen to change event of textTrackList and then for the 'showing'
track, listen to 'cuechange'.
This also means that captions can be turned on or off either via
programmatically changing the mode or doing it via the menu.
The menu doesn't currently reflect changes done programmatically.