Fix re-parenting TLWs in wxMSW

We need to set the new owner for the TLW, instead of using the new
parent as the actual parent, in the MSW sense, as this results in a
weird situation in which the TLW becomes a child (i.e. non-TLW) window.

Closes #18785.

include/wx/msw/nonownedwnd.h

@@ -22,6 +22,7 @@ public:
     wxNonOwnedWindow();
     virtual ~wxNonOwnedWindow();
 
+    virtual bool Reparent(wxWindowBase* newParent);
     virtual void InheritAttributes() wxOVERRIDE;
 
 protected: