Add wxTaskBarIcon::GetPopupMenu()

This is similar to CreatePopupMenu(), but the menu pointer returned by
the new function won't be deleted by wxWidgets, allowing it to return
the same pointer every time it is called.

Closes #18886.

include/wx/taskbar.h

@@ -62,6 +62,9 @@ protected:
     // creates menu to be displayed when user clicks on the icon
     virtual wxMenu *CreatePopupMenu() { return NULL; }
 
+    // same as CreatePopupMenu but won't destroy the menu
+    virtual wxMenu *GetPopupMenu() { return NULL; }
+
 private:
     // default events handling, calls CreatePopupMenu:
     void OnRightButtonDown(wxTaskBarIconEvent& event);