Added Subtitle Track selector

[TimGels]

Implemented the ability to select a subtitle track on the video player page.

Added a SubtitleAudioTrackSelector UserControl. This UserControl hosts the subtitle tracks and will also host the audio tracks from which a user can select a track.

Introduced a MediaTrackService which can be injected to provide some methods. Methods it provide are at this moment the retrieval of subtitle tracks and the preferred subtitle track. Due to LibVLC not being able to be injected by a dependency injector due to several reasons, this service needs to be initialized by calling the initialize method and providing it with a reference to a (initialized!) MediaPlayer instance. This service can be expanded to perform more general MediaPlayer tasks as well in the future if need be.

Added a ValueConverterGroup enabling the chaining of value converters.

Added a custom exception which gets thrown when one of the the MediaTrackService methods get called without the service being initialized.

Added MediaTrackService to the dependency Injector container in App.xaml.cs.

resolves #86