Changed the way unchecking the default box behaves.

* See the comment on setDefault().
Cleaned up set-default checkbox behavior.

- setDefault() now stores previous default time in var defaultTime when the checkbox is checked. If checkbox is unchecked without changing input time, the default time is set to its previous value.
- fixInput() now unchecks the checkbox if it was checked and the timer input is changed. setDefault() is not called in this process so the current default time will not change.