Toggle task marks on selected lines, as follows:
- If a line is checked, it will become unchecked.
- If a line is unchecked, it will become checked.
- If a (non-blank) line has no checkbox, it will be given one (unchecked), along with a list-hyphen if necessary.
- Blank lines will be left untouched.
(The built-in version of this action duplicates list-hyphens on lines without a checkbox, which annoyed me.)