Rename No frontend to Null frontend
Show a fadeout when the game ends and restarts
Make the restart fadeout faster
Move key-command mapping from game rules to client content
Remove experimental game modes from the main menu

They can eaily be invoked via commandline now, using the Makefile.
Don't intelligently share one frontend among many clients; KISS

This let's us simplify code structure greatly, at the cost
using multiplayer only for testing for now.
In the future, with web frontend and clients connecting via net,
multiplayer will be back, without sharing frontends.
Let clients spawn their own frontends
Rename things to reflect that frontend server keys to its client