More border rationalisation. GetDefaultBorder is now mostly defined in base class files.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@49893 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/bookctrl.h

@@ -243,6 +243,9 @@ protected:
         SetSelection_SendEvent = 1
     };
 
+    // choose the default border for this window
+    virtual wxBorder GetDefaultBorder() const { return wxBORDER_NONE; }
+
     // set the selection to the given page, sending the events (which can
     // possibly prevent the page change from taking place) if SendEvent flag is
     // included