Method providesSourceTitle added to FeedsParser

When an importer is attached to a content type, a node should be created to initiate an import. The title field for this node was always optional; if left empty Feeds would try to retrieve the title from the provided source. However, it depends on the implementation of the parser if providing such title is possible. For example, the CSV parser does not support providing a title for the feed node.

To solve this problem, a method called providesSourceTitle() has been added to the FeedsParser class. Parsers that support generating a title for the feed node, should implement this method and return TRUE. Parsers that do not support this, don't have to change anything. FeedsParser::providesSourceTitle() returns FALSE, so by default it is expected that parsers do not support this feature.

When a parser does not support providing a title, the title field on a feed node will now be required instead of optional (just like is the case for any other content type).

Example:

/**
 * Overrides FeedsParser::providesSourceTitle().
 *
 * This parser supports retrieving a title from the source.
 */
public function providesSourceTitle() {
  return TRUE;
}