IDEA - Shared Menus

[Jake-Moore]

Shared Menus

Implemented as a separate class with the goal of creating a menu to be shared among several players.

• Support for opening the same GUI for 2+ players where they all see the same thing. (Even allowing items to be placed into the shared GUI or updated for all viewers)
• The SharedMenu.Builder#build(...) method probably needs to not take a player, since this menu will be opened for multiple players. (This differs from the standard api in SimpleMenu & PaginatedMenu)

I am unsure how I want to implement this feature. One option is to simply show the exact same inventory for all viewers. Alternatively, we could create an inventory view per-player allowing additional player-specific data (like title or item descriptions) to be derived.