Add filter for which pages to load the spellchecker on.

I'm in the middle of writing a new post editor for WordPress and of course I can't use the built in post.php or post-new.php files to do the work so I've added a submenu items in each of the post types.  However this means that the spellchecker doesn't load on edit.php as TSpell_should_load_on_page() doesn't include it in the list of pages.

The merge request first updates all the code to the 1.3 release from WordPress.org and then adds a filter to allow other plugins to alter the pages list.