Refactoring of UnisonDB

Each table handler has its own class, implementing a common interface to ensure
basic table handling facilities.
For now, the calls to the UnisonDB methods have not been updated to use the per
table inner classes. But all the table specific methods have been set as
deprecated and schould thus no more be used. This will be updated soon, and the
deprecated methods will be removed.

Also, renames MusicItem to TrackItem.
Updates references to deprecated methods and removes deprecated methods

References to UnisonDB deprecated methods are updated to use the table specific
inner classes methods.
Organizes imports and formats project
Updates UnisonDB class javadoc

Finally removes MusicItem.java file (renamed in a previous commit)
Updates PlaylistLibraryService (under progress, not functional yet)

Starts and stops the service on login and logout.

Creates a new JsonStruct to handle the playlist deltas, because they don't fit
the existing Delta struct.

WARNING: Not functional yet, the deltas are not sent to the server.