Allow setting a default value for Alt and Title text in image fields

This is basically the same issue as https://www.drupal.org/node/807126 that was filed against the D6 branch of ImageField. That issue was closed as "fixed" by @quicksketch by pointing to the ImageField Tokens module:

Now with Tokens in core, it makes sense to take advantage of them and use them where possible.

PS: should support transliteration once in core (#904)

PR by @BWPanda: https://github.com/backdrop/backdrop/pull/3068