External and internal audio players work similarly now. Fixes #950.
* inherit a new ExternalAudioPlayer class from AudioPlayerInterface;
* use an existing ExternalViewer class to implement ExternalAudioPlayer;
* take (const char *, int) instead of std::vector<char> in
ExternalViewer constructor to fit into AudioPlayerInterface;
* extend ExternalViewer API to let ExternalAudioPlayer stop superseded
audio player processes;
* make AudioPlayerInterface::play() return an error message string to
allow reporting immediate failures from derived classes;
* Document AudioPlayerInterface API;
* Document AudioPlayerFactory::player();
* use the common audio interface exclusively in ArticleView.
