multiplayer stage 2: peer identity, stable vehicle ids and a session lobby

TTrain::id() numbers cabs with a local counter, in the order each peer happens
to create them, so it cannot address a vehicle across the session. Neither was
there any way for a player to see what is available to drive.

- network/entities: PeerId (the host is peer 1, remote peers are numbered from
  2, so it is a participant rather than a special case) and NetworkEntityId,
  plus a registry that numbers the drivable vehicles of the loaded scenario in
  name order and resolves ids back to TDynamicObject
- the server owns the numbering and publishes it as VEHICLE_LIST - name, crew
  occupancy, crew capacity, AI state, availability - on every change and at
  least every 120 frames, out of band so it stays out of the replay backbuffer;
  a client binds the received ids to its own copy of the scenario by name
- SERVER_HELLO hands the joining peer its PeerId
- a multiplayer lobby listing that roster, with vehicles taken by other players
  shown as such rather than hidden; entering one goes through the existing
  entervehicle command, the vehicle is never moved

The lobby lives inside the driver mode instead of a mode of its own: the world
has to keep running while somebody is choosing (a client stops consuming the
server's frames the moment its mode is not a command processor), and the same
screen is what a player needs to change vehicles later on.

Crew occupancy is still derived from the presence of a cab instance; the real
crew model, claim handshake and AI takeover follow in stage 3.

widgets/multiplayer_lobby.h

@@ -0,0 +1,35 @@
+/*
+This Source Code Form is subject to the
+terms of the Mozilla Public License, v.
+2.0. If a copy of the MPL was not
+distributed with this file, You can
+obtain one at
+http://mozilla.org/MPL/2.0/.
+*/
+
+#pragma once
+
+#include "application/uilayer.h"
+#include "input/command.h"
+
+namespace ui
+{
+
+// lobby of a multiplayer session: shows the vehicle roster published by the server and
+// lets the player pick the one to work on. it lives inside the driver mode rather than in
+// a mode of its own on purpose - the world has to keep running while somebody is choosing,
+// and the very same screen is what a player uses to change vehicles later on
+class multiplayer_lobby_panel : public ui_panel
+{
+public:
+	multiplayer_lobby_panel();
+
+	void render_contents() override;
+
+private:
+	void enter_vehicle(std::string const &Name);
+
+	command_relay m_relay;
+};
+
+} // namespace ui