While QSortFilterProxyModel emits layoutChanged when changing the
pattern, QSqlQueryModel emits modelReset. As only layoutChanged was
connected, a HistoryCategory would only work in a model that also had at
least one ListCategory.
The simplest solution is to have the parent model emit the signal
directly. This also emits a single signal on a pattern change rather
that one for each child model.
Resolves#3016.
40e4e73e36