At this point there are two filters:

* A grep filter that finds a pattern in a todo text
* A relevance filter, which selects todos that are relevant at this
  moment.

When setting a priority, a task may not be completed.
When a todo is completed, set priority to None.

A line is considered a todo item when it contains at least one
non-whitespace character.

Make sure the content is preserved byte by byte.

text() is the todo text without the tags. source() is the raw todo text.

A @ or + must be followed by at least one non-whitespace character to
qualify.