Extract translatables to registration

[xipasduarte]

When registering a post type of taxonomy, we could add something that would mark it for translation.

'ubb_translate' => true

This could make it easier to control what is translatable.

[goncaloasimoes]

I'm thinking this should be overridden by the values returned by the filter ubb_options.

If the filter return array has the key post_types and the value is an array (empty or otherwise), the post types/taxonomies set as translatable directly in their register data should be ignored.

The problem with this is setting the WordPress types/taxonomies as translatable (post, page, nav_menu, nav_menu_item, attachment, etc).