Game is now an interface, System is only a game detector.
MegaDiiveGame: set the version to the local country.
MegaDrive: removed all RomData references.
Library: replaced add_game(system, uri) by add(game).
SystemCollection: replaced game_found(system, uri) by game_found(game).
GameData: lots of changes.
GameData: added Version and Cover objects.
Use a Range for player number.
Add library usage doc(test) in README
Effectively use db_dir in Library's constructor