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.Shared/GameWindow/RequestWindowAttentionEvent.cs

@@ -0,0 +1,12 @@
+#nullable enable
+using System;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.GameWindow
+{
+    [Serializable, NetSerializable]
+    public sealed class RequestWindowAttentionEvent : EntityEventArgs
+    {
+    }
+}