Refactor Flammable to be ECS. (#4671)

- Refactor IHotItem into IsHotEvent.
- Refactor IFireAct into TileFireEvent.

Content.Shared/Atmos/FireVisuals.cs

@@ -0,0 +1,12 @@
+using System;
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.Atmos
+{
+    [Serializable, NetSerializable]
+    public enum FireVisuals
+    {
+        OnFire,
+        FireStacks,
+    }
+}