App: add hiddenref() expression built-in function

Any object reference inside this function is treated as hidden to
exclude it from dependency calculation. This function allows some form
of cyclic depdenency.

Merger note: renamed from "HREF" to "HIDDENREF" to avoid confusion with
the standard "hypertext reference" use of HREF.

src/App/PropertyExpressionEngine.h

@@ -85,9 +85,11 @@ public:
 
     struct ExpressionInfo {
         std::shared_ptr<App::Expression> expression; /**< The actual expression tree */
+        bool busy;
 
         ExpressionInfo(std::shared_ptr<App::Expression> expression = std::shared_ptr<App::Expression>()) {
             this->expression = expression;
+            this->busy = false;
         }
 
         ExpressionInfo(const ExpressionInfo & other) {
@@ -186,6 +188,10 @@ private:
                 boost::unordered_map<int, App::ObjectIdentifier> &revNodes, 
                 DiGraph &g, ExecuteOption option=ExecuteAll) const;
 
+    void slotChangedObject(const App::DocumentObject &obj, const App::Property &prop);
+    void slotChangedProperty(const App::DocumentObject &obj, const App::Property &prop);
+    void updateHiddenReference(const std::string &key);
+
     bool running; /**< Boolean used to avoid loops */
     bool restoring = false;
 
@@ -201,6 +207,9 @@ private:
     /**< Expressions are read from file to this map first before they are validated and inserted into the actual map */
     std::unique_ptr<std::vector<RestoredExpression> > restoredExpressions;
 
+    struct Private;
+    std::unique_ptr<Private> pimpl;
+
     friend class AtomicPropertyChange;
 
 };