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/T3D/fx/explosion.h

@@ -205,6 +205,7 @@ class Explosion : public GameBase, public ISceneLight
    void setCollideType( U32 cType ){ mCollideType = cType; }
 
    DECLARE_CONOBJECT(Explosion);
+   DECLARE_CATEGORY("UNLISTED");
    static void initPersistFields();
 private:
    SimObject*     ss_object;