moved code working around combobox selection bug to wxComboBox: wxChoice doesn't have selection anyhow, why should it be there?

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@31540 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/combobox.h

@@ -136,6 +136,9 @@ public:
 
 protected:
     virtual WXDWORD MSWGetStyle(long style, WXDWORD *exstyle) const;
+    virtual void DoSetSize(int x, int y,
+                           int width, int height,
+                           int sizeFlags = wxSIZE_AUTO);
 
     // common part of all ctors
     void Init() { m_selectionOld = -1; }