Toggle completed to-dos back to incomplete

If a non-recurring to-do has been completed, selecting "Toggle To-do
Completed" from its right-click menu does not make it incomplete, because
`Todo::setPercentComplete(0)` is not sufficient.  This patch brute-forces
all completion-related fields.

CCBUG: 397721
1 job for release/21.08 in 9 minutes and 36 seconds (queued for 176 minutes and 12 seconds)
Status Name Job ID Coverage
passed build_clazy_clang_tidy #117037