Generalize wxBookCtrlBase::m_bookctrl to wxWindow

The field is used for common placement and sizing logic in
notebook-like controls that have a "controller buddy" control. Its use
isn't wxControl-specific and only relies on wxWindow API, so the
restriction to wxControl-inherited types is arbitrary.

Change the type to the more general wxWindow* pointer.

In practical terms, this allows derived classes to use e.g.
wxNativeWindow as the controller buddy.

include/wx/bookctrl.h

@@ -339,7 +339,7 @@ protected:
     void OnSize(wxSizeEvent& event);
 
     // controller buddy if available, NULL otherwise (usually for native book controls like wxNotebook)
-    wxControl *m_bookctrl;
+    wxWindow *m_bookctrl;
 
     // Whether to shrink to fit current page
     bool m_fitToCurrentPage;