Turns GameTicker into an EntitySystem. (#4197)

* GameTicker turned into an EntitySystem

* Turns ClientGameTicker into an EntitySystem, turn NetMessages into events

* Change event names to be more consistent with the rest.

* YAML linter uses the dummy gameticker CVar override.

* Fix game ticker initialization order

* Dummy ticker won't spawn players.

* Fix character creation test

Content.Server/GameTicking/GameTicker.LobbyMusic.cs

@@ -0,0 +1,78 @@
+using Content.Shared.Audio;
+using Robust.Shared.ContentPack;
+using Robust.Shared.IoC;
+using Robust.Shared.Log;
+using Robust.Shared.Random;
+using Robust.Shared.Utility;
+using Robust.Shared.ViewVariables;
+
+namespace Content.Server.GameTicking
+{
+    public partial class GameTicker
+    {
+        [Dependency] private readonly IResourceManager _resourceManager = default!;
+        private const string LobbyMusicCollection = "LobbyMusic";
+
+        [ViewVariables]
+        private bool _lobbyMusicInitialized = false;
+
+        [ViewVariables]
+        private SoundCollectionPrototype _lobbyMusicCollection = default!;
+
+        [ViewVariables]
+        public string? LobbySong { get; private set; }
+
+        private void InitializeLobbyMusic()
+        {
+            DebugTools.Assert(!_lobbyMusicInitialized);
+            _lobbyMusicCollection = _prototypeManager.Index<SoundCollectionPrototype>(LobbyMusicCollection);
+
+            // Now that the collection is set, the lobby music has been initialized and we can choose a random song.
+            _lobbyMusicInitialized = true;
+
+            ChooseRandomLobbySong();
+        }
+
+        /// <summary>
+        ///     Sets the current lobby song, or stops it if null.
+        /// </summary>
+        /// <param name="song">The lobby song to play, or null to stop any lobby songs.</param>
+        public void SetLobbySong(string? song)
+        {
+            DebugTools.Assert(_lobbyMusicInitialized);
+
+            if (song == null)
+            {
+                LobbySong = null;
+                return;
+                // TODO GAMETICKER send song stop event
+            }
+
+            if (!_resourceManager.ContentFileExists(song))
+            {
+                Logger.ErrorS("ticker", $"Tried to set lobby song to \"{song}\", which doesn't exist!");
+                return;
+            }
+
+            LobbySong = song;
+            // TODO GAMETICKER send song change event
+        }
+
+        /// <summary>
+        ///     Plays a random song from the LobbyMusic sound collection.
+        /// </summary>
+        public void ChooseRandomLobbySong()
+        {
+            DebugTools.Assert(_lobbyMusicInitialized);
+            SetLobbySong(_robustRandom.Pick(_lobbyMusicCollection.PickFiles));
+        }
+
+        /// <summary>
+        ///     Stops the current lobby song being played.
+        /// </summary>
+        public void StopLobbySong()
+        {
+            SetLobbySong(null);
+        }
+    }
+}