Unify _menu_attributes_form_alter() to not clobber any other attributes or options in the link data.

Issue #1132782: Added update function to fix any menu links that had the class attribute saved as a string rather than an array.

#994402 by das-peter, Dave Reid: Fixed Prevent error in menu_attributes_menu_link_alter() if an attribute is not a string.

by Dave Reid: Moved the description element handling back into menu_attributes_form_menu_edit_item_alter() and improved display of attribute form elements.

Fixed #372355 (Check for existance of menu settings in node edit form)
Added #363681 (Add administration page to allow administrator to set which attributes can be configured and their default values)

Added accesskey attribute: http://drupal.org/node/358981

Removed tab characters (Issue: 352737). Thanks to AlexisWilke