Allow title callbacks to return unsanitized text (after being run through t())

Describe your issue or idea

In many places in Backdrop we use title callbacks in hook_menu() to set the page titles, while in others we use backdrop_set_title(). The title callback approach has some advantages: it affects breadcrumbs and menu links for example. While the backdrop_set_title() approach is convenient it only affects the current page.

While attempting to use title callbacks instead of backdrop_set_title(), I found that it was not possible to use the t() function and return text that used the placeholder replacement (i.e. t('Edit %title')). Instead you end up with a title that displays with the HTML escaped, such as this:

It would be a good thing if we could make it so that title callbacks could provide already-escaped text, so that we could include HTML in the page titles, as is possible with backdrop_set_title().

PR https://github.com/backdrop/backdrop/pull/2098