CategoryPopupWidget: Track popup hiding again so sortkey changes are applied

OOjs UI killed the 'hide' event that was running this code in the dialog refactor
back in July (Id121fc3a), but left it documented (along with 'show').

Just use the 'toggle' event instead.

Bug: 72168
Change-Id: I172fac81c4cbb89505522011aed57db57ccbc52f
Merge "CategoryPopupWidget: Track popup hiding again so sortkey changes are applied"