Add discord-rpc

[Deihim007]

Based on #2499 Prior discussions/implementations #721, #1031, #1570

This PR adds basic discord-rpc features to the client. (Other features will be added later on a separate PR)

Todo:

• [x] Todolist
• [x] Add discord-rpc
  • [x] Installation script
  • [x] dependencies (rapidjson)
• [x] State:
  • [x] Main Menu
  • [x] In-game
    • [x] Elapsed time

Maybe:

• [ ] Default game-mode implementation
  • [ ] Idle (Custom state)
  • [ ] Loading
  • [ ] Player count ?
• [ ] Custom Application Id
  • [ ] Server-side function
  • [ ] Client settings option to disable it (prevent potential abuse)
  • [ ] Wiki tutorial
    • [ ] How to make an application on discord developer portal
      • [ ] Assets template ?