Introduce a nav menu title filter

[paulwilde]

Currently the only way to filter the output of a nav menu title is to use the the_title filter. This isn't ideal as this is also the filter used for the_title(). It also doesn't include any of the nav_menu specific arguments that are exposed inside the start_el() method except for the ID of the menu item.

I've attached a patch which introduces a separate filter for the title which includes the same arguments used by nav_menu_css_class, nav_menu_item_id, nav_menu_link_attributes and walker_nav_menu_start_el for consistency.

[kucrut]

I think the filter should be named nav_menu_item_title instead to avoid confusion with the menu title/name.

[johnbillion]

Unfortunately we have a mixture of nav_menu_* and nav_menu_item_* for our nav menu item filter names. nav_menu_item_title would be better.

[johnbillion]

In 34666:

Add a nav_menu_item_title filter for filtering nav menu item titles.

Fixes #33447
Props paulwilde