Added a class to manage automatically the sidebar objectives.

Recalculating the score of the entries to have them sorted in the good order, with small scores.

(Also, updated the imports.)
Moved the ScoreboardManager out of the GameManager.
The entries are truncated at 16 characters (and the display name at 32), to respect
Minecraft limitations.

Also, fixed the removeEntry() method.
Refactorization of the scoreboard: the plugin now uses the new UHSidebarObjective utility.

Also, a few bugfixes of the previous commit.
Bugfix: players count not updated if the game is started without players and players are "resurrected" after.