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/combobox.h

@@ -139,6 +139,8 @@ class WXDLLIMPEXP_CORE wxComboBox :
     virtual wxTextWidgetImpl* GetTextPeer() const;
 #endif // wxOSX_USE_CARBON
 
+    virtual void Popup();
+    virtual void Dismiss();
 
 
     // osx specific event handling common for all osx-ports