HIG seperate primary and secondary text by 24pixels

"Add one line break at the standard font size below both the primary and
secondary text, or 24 pixels if you are using Glade"

Change-Id: I4bdb0eade4de1304a155fe4338c013d1576b8282

vcl/source/window/layout.cxx

@@ -2021,6 +2021,7 @@ short MessageDialog::Execute()
         m_pGrid = new VclGrid(pContainer);
         m_pGrid->reorderWithinParent(0);
         m_pGrid->set_column_spacing(12);
+        m_pGrid->set_row_spacing(GetTextHeight());
 
         m_pImage = new FixedImage(m_pGrid, WB_CENTER | WB_VCENTER | WB_3DLOOK);
         switch (m_eMessageType)