ObjReference: Provide operator!=

Logical symmetry.

src/Editor/Undo/ObjReferences.h

@@ -80,6 +80,11 @@ public:
     {
         return (mNodeID == rkOther.mNodeID && mpScene == rkOther.mpScene);
     }
+
+    bool operator!=(const CNodePtr& other) const
+    {
+        return !operator==(other);
+    }
 };
 
 class CInstancePtr
@@ -109,6 +114,11 @@ public:
     {
         return (mInstanceID == rkOther.mInstanceID && mpArea == rkOther.mpArea);
     }
+
+    bool operator!=(const CInstancePtr& other) const
+    {
+        return !operator==(other);
+    }
 };
 
 class CLinkPtr
@@ -137,6 +147,11 @@ public:
     {
         return (mpInstance == rkOther.mpInstance && mLinkIndex == rkOther.mLinkIndex);
     }
+
+    bool operator!=(const CLinkPtr& other) const
+    {
+        return !operator==(other);
+    }
 };
 
 DEFINE_PTR_LIST_CLASS(CNodePtrList, CNodePtr, CSceneNode*)