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.

application/driveruilayer.cpp

@@ -37,6 +37,7 @@ driver_ui::driver_ui()
 	add_external_panel(&m_logpanel);
 	add_external_panel(&m_perfgraphpanel);
 	add_external_panel(&m_cameraviewpanel);
+	add_external_panel(&m_multiplayerlobby);
 	m_logpanel.is_open = false;
 
 	m_aidpanel.title = STR("Driving Aid");
@@ -87,10 +88,18 @@ void driver_ui::render_menu_contents()
 		if (ImGui::MenuItem(m_timepanel.name().c_str()))
 			m_timepanel.open();
 
+		if (Application.is_server() || Application.is_client())
+			ImGui::MenuItem(m_multiplayerlobby.name().c_str(), nullptr, &m_multiplayerlobby.is_open);
+
 		ImGui::EndMenu();
 	}
 }
 
+void driver_ui::show_multiplayer_lobby(bool const Show)
+{
+	m_multiplayerlobby.is_open = Show;
+}
+
 void driver_ui::showDebugUI()
 {
 	m_debugpanel.is_open = !m_debugpanel.is_open;