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/generic/renderg.cpp

@@ -67,6 +67,8 @@ public:
 
     virtual int GetHeaderButtonHeight(wxWindow *win);
 
+    virtual int GetHeaderButtonMargin(wxWindow *win);
+
     virtual void DrawTreeItemButton(wxWindow *win,
                                     wxDC& dc,
                                     const wxRect& rect,
@@ -445,6 +447,11 @@ int wxRendererGeneric::GetHeaderButtonHeight(wxWindow *win)
     return h + d + 2 * HEADER_OFFSET_Y + EXTRA_HEIGHT;
 }
 
+int wxRendererGeneric::GetHeaderButtonMargin(wxWindow *WXUNUSED(win))
+{
+    return 5;
+}
+
 
 // draw the plus or minus sign
 void