With an example of Blink

Use the Entity class everywhere, instead
fix #2803

The world is now isometric. fix #2277
Simplify TurnHandler. fix #2724 #2704

The world and game modules are now more decoupled. fix #2587

Implement a debughud. fix #2417

I just moved a class out of the game module, because it required sfml (for
the gui callbacks), into the gui module. Also renamed it.  And also, remove
tests/game_*main.cpp, we now use only src/main/batajelo_{client|server}.cpp
to compile the binaries.

Since doing more than one simultaneous async_write on one single socket
is NOT safe, we use a deque<Command*> to keep the list of all the command
to send, and we send them properly one by one.

This makes the data easier to track and free after
they were sent. And it makes sure the buffer stays valid
for all the async_write() process.
It’s also easier to send data without having to bother with
creating a string, parsing everything, etc.

TODO: the reception of that file by the client.

Implement all the connection + auth process. Also fix a leak by correctly deleting the clients that get disconnected.

Do all includes with <> instead of "". We just need to pass the -Isrc/ option to the compiler, or equivalent.

Now we can easily implement a request/answer protocol between the clients
and the server.