We already know which list widget a `<th>`
tag is associated with when we set up the
event handler, so it's silly to read data
from the DOM to find that widget again
when the handler runs.
This commit eliminates a whole class of possible
errors and busy work.
Giving these functions a name and moving them to
the top-level scope has a couple tactical advantages:
- names show in tracebacks
- code is less indented
- setup code is less cluttered
- will be easier to add unit tests
- will make some upcoming diffs nicer
These are technically more `compare_foo` than `sort_foo`,
but we already had a naming convention that was sort of
in place.
Tim Abbott
Steve Howell
Thomas Ip
Anders Kaseorg