Fix mechanism events not being called properly, add test (#2279)

* Add mechanism events when added/removed to/from body/parts

* Change old usages

* Add TODO

* Remove BodyExtensions and IHasBody

* Remove unnecessary extensions and fix wrong event call in mechanism behavior component

* Complete test and fix event calls

Content.Client/GameObjects/Components/Mobs/HumanoidAppearanceComponent.cs

@@ -47,7 +47,7 @@ namespace Content.Client.GameObjects.Components.Mobs
                 return;
             }
 
-            if (Owner.TryGetBody(out var body))
+            if (Owner.TryGetComponent(out IBody body))
             {
                 foreach (var part in body.Parts.Values)
                 {