[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/Gui/ViewProviderBalloon.h

@@ -58,6 +58,9 @@ public:
     virtual std::vector<std::string> getDisplayModes(void) const;
     virtual void updateData(const App::Property*);
     virtual void onChanged(const App::Property* p);
+    virtual bool setEdit(int ModNum);
+    virtual void unsetEdit(int ModNum);
+    virtual bool doubleClicked(void);
 
     virtual TechDraw::DrawViewBalloon* getViewObject() const;
 };