Rework commands

[Mindgamesnl]

I started working on the preload/unload commands when I realized how convoluted some of the current command handling is.

This PR contains the following core changes:

• Player selectors are now part of the Platform
• the User interface can now inherit the actual type of its parent (Player, CommandSender, CommandSource, etc.)
• the SpigotPlayerSelector, BungeePlayerSelector, and VelocityPlayerSelector now all implement one interface (SelectorTranslator<>`), which is platform agnostic, though some specific platform features can still be accessed through its T param.
• Some commands are essentially re-implemented per platform and can now carry a single implementation, this affects
  • /oa play
  • /oa stop
  • /audio (and aliasses)
  • .. and new commands implemented from here on out