Add wxComboBox::Popup() and Dismiss() to manually show or hide its popup.

Add implementations for wxMSW and wxGTK.

Closes #11506.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63242 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/combobox.h

@@ -88,6 +88,8 @@ public:
     {
         return wxItemContainer::GetStringSelection();
     }
+    virtual void Popup();
+    virtual void Dismiss();
 
     virtual void Clear()
     {