[TD] Add TaskPanel for Balloons

TaskPanel can be open by double clicking either Balloon label on drawing
page or Balloon object in tree view.

src/Mod/TechDraw/App/DrawViewBalloon.h

@@ -73,13 +73,14 @@ public:
         return "TechDrawGui::ViewProviderBalloon";
     }
 
+    static const char* balloonTypeEnums[];
+    static const char* endTypeEnums[];
+
 protected:
     void onChanged(const App::Property* prop);
     virtual void onDocumentRestored();
 
 private:
-    static const char* endTypeEnums[];
-    static const char* balloonTypeEnums[];
 };
 
 } //namespace TechDraw