Add wxWindow::MSWAfterReparent() virtual function

Refactor the existing code to extract the part of Reparent() updating
the new parent style to an overridable virtual function.

No real changes yet.

include/wx/msw/window.h

@@ -624,6 +624,11 @@ protected:
     virtual bool DoPopupMenu( wxMenu *menu, int x, int y ) override;
 #endif // wxUSE_MENUS_NATIVE
 
+    // Called by Reparent() after the window parent changes, i.e. GetParent()
+    // returns the new parent inside this function.
+    virtual void MSWAfterReparent();
+
+
     // the window handle
     WXHWND                m_hWnd;
 

src/msw/window.cpp

@@ -1683,12 +1683,17 @@ bool wxWindowMSW::Reparent(wxWindowBase *parent)
 
     ::SetParent(hWndChild, hWndParent);
 
+    MSWAfterReparent();
+
+    return true;
+}
+
+void wxWindowMSW::MSWAfterReparent()
+{
     if ( wxHasWindowExStyle(this, WS_EX_CONTROLPARENT) )
     {
         EnsureParentHasControlParentStyle(GetParent());
     }
-
-    return true;
 }
 
 static inline void SendSetRedraw(HWND hwnd, bool on)