clean up "Add" menubar categoes on the 3d object side via the following:

SceneObject now defaults to a category of "misc"
getCategoryOfClass now checks parent classes for what categories they hold so that variants can inherit
categories are in one of a few rough groups and subgroups depending on actual mapper needs/usages

Engine/source/environment/skyBox.h

@@ -75,6 +75,7 @@ public:
    virtual ~SkyBox();
 
    DECLARE_CONOBJECT( SkyBox );
+   DECLARE_CATEGORY("Environment \t Background");
 
    // SimObject
    void onStaticModified( const char *slotName, const char *newValue );
@@ -129,4 +130,4 @@ protected:
    BaseMatInstance* _getMaterialInstance();
 };
 
-#endif // _SKYBOX_H_
+#endif // _SKYBOX_H_