first draft of respawn

ToDo

• Client Part
  • add a floating window similar to the login or quit window
  • add a unique button to respawn
  • plug this button pressed signal to a network command to respawn
  • add network command to respawn
  • Format of deadmessages and wether we should localize the messages
    • should keep the origin comments, json does not have comments
• Server Part
  • add a PlayerAgent::Respawn that trigger a WorldAgent::PopAgent and WorldAgent::PushAgent functions without passing by AddAgent/RemoveAgent

Remaining bugs:

There is some race condition that sometimes happens on respawn: Invalid type in function 'GetDamageRatio' in base 'CharacterBody2D (PlayerAgent)'. The Object-derived class of argument 1 (previously freed) is not a subclass of the expected argument class.