Added CanApplyParentThemeBackground and use it in wxMSW's

wxNotebook::ApplyThemeBackground.  It's in the common base classes so
that someday it's usage can expand to the other platforms as well.


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@26085 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/popupwin.h

@@ -46,6 +46,8 @@ public:
     virtual void Position(const wxPoint& ptOrigin,
                           const wxSize& size);
 
+    virtual bool IsTopLevel() const { return TRUE; }
+
     DECLARE_NO_COPY_CLASS(wxPopupWindowBase)
 };