Some of the Object::equals now are const correct.

There's more to do.

Why make this const-correct, instead of ditch it?  Because whatever
I replace it with has to be ready for const correctness.

dw/iterator.hh

@@ -30,8 +30,8 @@ private:
    Content::Type mask;
 
 public:
-   bool equals (Object *other);
-   void intoStringBuffer(lout::misc::StringBuffer *sb);
+   bool equals (const Object *other) const override;
+   void intoStringBuffer(lout::misc::StringBuffer *sb) const override;
 
    inline Widget *getWidget () { return widget; }
    inline Content *getContent () { return &content; }