Add wxRenderer::GetHeaderButtonMargin().

Used for best size calculations of column width.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65947 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/renderer.cpp

@@ -56,6 +56,8 @@ public:
 
     virtual int GetHeaderButtonHeight(wxWindow *win);
 
+    virtual int GetHeaderButtonMargin(wxWindow *win);
+
 
     // draw the expanded/collapsed icon for a tree control item
     virtual void DrawTreeItemButton(wxWindow *win,
@@ -226,6 +228,12 @@ int wxRendererGTK::GetHeaderButtonHeight(wxWindow *WXUNUSED(win))
     return req.height;
 }
 
+int wxRendererGTK::GetHeaderButtonMargin(wxWindow *WXUNUSED(win))
+{
+    wxFAIL_MSG( "GetHeaderButtonMargin() not implemented" );
+    return -1;
+}
+
 
 // draw a ">" or "v" button
 void