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.Shared/GameObjects/Components/Body/Part/SharedBodyPartComponent.cs

@@ -114,15 +114,6 @@ namespace Content.Shared.GameObjects.Components.Body.Part
             mechanism.Part = this;
             SizeUsed += mechanism.Size;
 
-            if (Body == null)
-            {
-                mechanism.AddedToPart();
-            }
-            else
-            {
-                mechanism.AddedToPartInBody();
-            }
-
             Dirty();
         }
 
@@ -132,15 +123,6 @@ namespace Content.Shared.GameObjects.Components.Body.Part
             mechanism.Part = null;
             SizeUsed -= mechanism.Size;
 
-            if (Body == null)
-            {
-                mechanism.RemovedFromPart(this);
-            }
-            else
-            {
-                mechanism.RemovedFromPartInBody(Body, this);
-            }
-
             Dirty();
         }