This adds a feature similar to the org-fontify-done-headline, but
instead for the TODO headlines.

This is enabled with the boolean customization org-fontify-todo-headline
and can be themed via the face org-headline-todo.

I was missing this when I wanted to distinguish more clearly between the
TODO headlines and the other headlines (more so than the highlighted
TODO keyword(s)).

Hope someone else finds this useful.

Best regards,
Terje Larsen