Renaming of audioTrackNumber and numberOfAudioTracks

[JuergenGrupp]

The two properties audioTrackNumber and numberOfAudioTracks use the term number in different meanings. We need to clarify this by renaming them to hasAudioTrackIndex and hasTotalNumberOfAudioTracks. Additionally both properties take only positive integers, so their data type should become xsd:nonNegativeInteger.
The same applies to videoTrackNumber and numberOfVideoTracks: We should rename them to hasVideoTrackIndex and hasTotalNumberOfVideoTracks
This issue replaces #343 by @MarcAntoine-Arnaud

[JuergenGrupp]

Another concern: Which classes should carry the ...Index and the ..TotalNumberOf.. properties? Here's my proposal:
The ...Index properties are describing the AudioTrack and the VideoTrack directly and should be placed in subclasses of Track, and defined as subproperties of a trackIndex property in the class Track.
Whereas the ...TotalNumberOf... properties should be defined in MediaRessource as the object that contains the countable tracks.