Fix setting colours for the edit control part of wxComboBox.

Recognize this control as part of wxComboBox and so handle WM_CTLCOLOR for it.

To do this, override ContainsHWND() in wxComboBox and use it, on the parent
window, in wxControl::DoMSWControlColor(), if we fail to find the window
directly.

Closes #811.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72468 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/combobox.h

@@ -92,6 +92,7 @@ public:
     virtual void SetSelection(long from, long to)
         { wxTextEntry::SetSelection(from, to); }
     virtual int GetSelection() const { return wxChoice::GetSelection(); }
+    virtual bool ContainsHWND(WXHWND hWnd) const;
     virtual void GetSelection(long *from, long *to) const;
 
     virtual bool IsEditable() const;