GUI: improve debug menu

src/engine/dispatch.h

@@ -292,6 +292,12 @@ class DivDispatch {
      */
     virtual void poke(std::vector<DivRegWrite>& wlist);
 
+    /**
+     * get available registers.
+     * @return an array of C strings, terminated by NULL; or NULL if none available.
+     */
+    virtual const char** getRegisterSheet();
+
     /**
      * initialize this DivDispatch.
      * @param parent the parent DivEngine.