dhtml_menu 6.x-2.0

#257142: Conflicting branches have been merged. This version does not significantly change functionality as it just incorporates previous changes from the stable branch, but it fixes coding style and adds an update path for users upgrading from a version of DHTML Menu that still clones menu blocks (pre 6.x-1.04).

These changes in the stable branch have been ported:
- #257273: Add DHTML to existing menu blocks rather than cloning them
- #257296: Split code into include files
- #257362: Readme file updated
- #222303: Clean the id attribute by white-listing allowed characters, not an incomplete black-list.
- #257284: All menus use DHTML by default
- #257138: admin/settings/dhtml_menu is accessible again (compatibility problem with Drupal 6.2)