Gui: Add getObject<T>() helpers to various classes

Obtaining specific kind of document object is very common task. This
commit introduces handy helper which makes that a lot easier.

src/Mod/TechDraw/App/DimensionReferences.h

@@ -63,6 +63,7 @@ public:
     bool operator== (const ReferenceEntry& otherRef) const;
 
     App::DocumentObject* getObject() const;
+    template <class T> T* getObject() const { return dynamic_cast<T*>(getObject()); }
     void setObject(App::DocumentObject* docObj) { m_object = docObj; }
     std::string getSubName(bool longForm = false) const;
     void setSubName(const std::string& subName) { m_subName = subName; }