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/html/htmlwin.h

@@ -198,6 +198,8 @@ public:
     void SelectAll();
 #endif
 
+    virtual bool CanApplyParentThemeBackground() const { return false; }
+    
 protected:
     void Init();