media player: add selector to query for the number of chapters, rename previous title count selector

VLCMedia: added new parsing method which allows differenciated parsing and fetching of local and remote sources

VLCMedia: add new keys for total track number, director, season, episode, show name, actors, album artist, disc number

(cherry picked from commit 33f96d81)

VLCMedia: add delegate method called when media's meta data is changed independently if this was done by preparsing or not

VLCMediaPlayer: mark implementation of mediaPlayerTimeChanged as optional as listing for the notification is fine, too

VLCMediaPlayer: don't pause playback if the iOS app is going to the background, since this behavior may be different per client application