MLAlbumTrack: add API to allow album track (and in turn album) creation without knowledge about the track number

only save relative paths on iOS, save full paths on OS X
migrate full urls strings to relative path strings
add url property to MLFile which always returns full absolut NSURL

MLAlbumTrack:
+ containsArtwork
MLFile:
+ thumbnailName

add unread property in model implemented in the class but not added to the model
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

move library migration code to category - remove old GreatSharkHuntDatabaseFormat migration which is no longer needed - in debug mode clean new base path before doing the migration - remove all sqlite remaining from old database path
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

on device category 3 and later, save thumbnails as JPEG instead of PNG since those devices include hardware acceleration to handle the (de)compression

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

add a property for library base path which is publicly accessable add a method to migrate the database and thumbnails to a new library base path
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

(cherry picked from commit 158f9d75)
Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>