libnotificationmanager: Fix infinite recursion when adding a second applet
Add a QConcatenateTablesProxyModel subclass which always has 1 column.
This is a workaround to https://bugreports.qt.io/browse/QTBUG-134210 and https://bugs.kde.org/show_bug.cgi?id=500749 if a model is added to an empty QConcatenateTablesProxyModel it will report zero columns and non-zero rows, causing a data inconsistence which causes an infinite recursion in NotificationFilterProxyModel::filterAcceptsRow remove when the upstream bug is fixed
Edited by Marco Martin