* add a new interface class AudioPlayerInterface;
* inherit a new proxy class Ffmpeg::AudioPlayer from it;
* partially switch ArlticleView to using the interface;
* expose MainWindow's AudioPlayerInterface instance to all ArticleView
instances;
* add a new AudioPlayerFactory class responsible for creating instances
of concrete classes derived from AudioPlayerInterface depending on
relevant Config::Preferences values;
* increase minimum supported Qt version from 4.5 to 4.6 in README
in order to use QScopedPointer introduced in Qt 4.6.
