code: public String getSortImagePath(SortDirection sortDirection) {
          /* todo: move to the DataGridConfig object */
          if(sortDirection == SortDirection.ASCENDING)
              return getMessage(IDataGridMessageKeys.SORT_ASC_IMAGE_PATH);
          else if(sortDirection == SortDirection.DESCENDING)
              return getMessage(IDataGridMessageKeys.SORT_DESC_IMAGE_PATH);
          else {
              assert sortDirection == SortDirection.NONE : "Encountered an invalid sort direction.";
              return getDefaultSortImagePath();
          }
      }
docs: <p>
  Get the image paths used for the given {@link SortDirection}.  The image paths are discovered
  by using the following mapping.
  <br/>
  <table>
  <tr><td>Sort direction</td><td>Message key</td></tr>
  <tr><td><code>{@link SortDirection#ASCENDING}</code></td><td><code>{@link IDataGridMessageKeys#SORT_ASC_IMAGE_PATH}</code></td></tr>
  <tr><td><code>{@link SortDirection#DESCENDING}</code></td><td><code>{@link IDataGridMessageKeys#SORT_DESC_IMAGE_PATH}</code></td></tr>
  <tr><td><code>{@link SortDirection#NONE}</code></td><td><code>{@link IDataGridMessageKeys#SORT_NONE_IMAGE_PATH}</code></td></tr>
  </table>
  The value for the message is obtained by looking up a value in the {@link DataGridResourceProvider} obtained
  via {@link #getResourceProvider()} using the message key in the table above.
  </p>
  @param sortDirection the {@link SortDirection} used to lookup an image path
  @return the string image used to represent a sort direction graphically