Twenty Ten/Eleven: Use a filter to modify the output of wp_title()

[obenland]

This will help to clean up header.php as well as make the first two Twenty themes more compatible with the ​WPTRT guidelines:

Themes are required to modify output via filter (wp_title/body_class/post_class)

See twentytwelve_wp_title() in Twenty Twelve for example code.

In 23718:

Twenty Eleven: use a filter to modify the output of wp_title(). Props obenland, see #23774.

In 23719:

Twenty Ten: use a filter to modify the output of wp_title(). Props obenland, closes #23774.

Bad news bears. :(

This change isn't backward compatible with child themes. If a child theme creates its own header.php template file, the title can be duplicated or worse.

We'll need to revert.

Twenty Twelve and onward should be fine since they shipped with the filter from the beginning.

In 23778:

Twenty Ten and Twenty Eleven: revert r23720 and r23721 (closes #23772) and r23718 and r23719 (closes #23774).

Unfortunately, it's too late for these improvements in older themes since it's very likely they'll adversely affect child themes.