Fix possible use of dangling window pointer in wxSizer::Insert()

Add the sizer item to the sizers items list only after calling of
wxWindowBase::SetContainingSizer() because it can throw (if it asserts
and the assert handler throws an exception, as happens in our own unit
tests) and then the sizer item would be kept in the sizers items list
but m_containingSizer wouldn't be set for the window.

include/wx/sizer.h

@@ -321,6 +321,10 @@ public:
     // Enable deleting the SizerItem without destroying the contained sizer.
     void DetachSizer() { m_sizer = NULL; }
 
+    // Enable deleting the SizerItem without resetting the sizer in the
+    // contained window.
+    void DetachWindow() { m_window = NULL; m_kind = Item_None; }
+
     virtual wxSize GetSize() const;
     virtual wxSize CalcMin();
     virtual void SetDimension( const wxPoint& pos, const wxSize& size );