Complete the update path from Tao 6 to Tao 7

There is a fully outlined upgrade path available at http://drupal.org/node/254940
We should check every step has been followed in Tao port, since I noticed several steps missing.

• #1422958: Properly use classes_array, attributes_array, title_attributes_array and content_attributes_array
• #1423088: All theme functions now take a single argument, $variables
• #1423552: $left and $right variables are now $sidebar_first and $sidebar_second; CSS IDs also changed
• #1423654: theme_links() has a new parameter 'heading' for accessibility
• #1423664: theme_links() now allows more targeted overrides
• #1440932: Skip to main content links in core themes
• #1440992: A proper thumbnail screenshot
• #1440958: Browser-targeted CSS files can now be added on tao.info
• #1440968: PHPTemplate is now the default theme engine

This is the result of a thorough review of the upgrade path, but if you find anything else, please add to the list.