Updates PlaylistLibraryService (work in progress)

WARNING: the following changes haven't been tested, possible issues!

* Adds a field in the local app DB to store a copy of the local last date
modified.
* Adds methods to PlaylistItem to set/get date modified field.
* Updates the storage of a playlist to fill this date modified field.
* Updates the retrieval of a playlist to add the date modified field to the
PlaylistItem.

Adds a new API method for sending batch updates of playlist library (for every
users of the device at the same time).
Updates PlaylistLibraryService (WARNING: work in progress)

Also updates AbstractItem definition to enforce explicit types for specialized
classes.
Updates PlaylistLibraryService

Implements UnisonDB.Playlist.updateDateModified()
Updates PlaylistLibrary Service (minor changes)

Comments usage of tracks field (actually useless)

Renames PlaylistItem.size() to PlaylistItem.getSize() and vice-versa
Updates and FIXes some bugs with PlaylistLibraryService

FIX bugs with app DB handling

AndroidDB: adds method to retrieve a unique playlist with its data and tracks
Organizes imports and formats project (servercom not tested)

Note: server communication and api haven't been tested.