Implement wxComboBox::Popup() and Dismiss() for wxOSX/Cocoa.

Unlike in the other ports, these methods currently don't generate any events
under OS X because these events are never generated at all there.

Closes #12642.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69948 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/osx/core/private.h

@@ -690,6 +690,8 @@ public :
     virtual void RemoveItem(int WXUNUSED(pos)) {}
 
     virtual void Clear() {}
+    virtual void Popup() {}
+    virtual void Dismiss() {}
 
     virtual wxString GetStringAtIndex(int WXUNUSED(pos)) const { return wxEmptyString; }