[Notifications] Don't bind model inside headerItem

Since Qt 5.12.5 and newer this causes infinite recursion in component creation as setting the model has ListView recreate
its header item (among other things) since https://code.qt.io/cgit/qt/qtdeclarative.git/commit/?h=5.12&id=692b2da77427259a3589cf8a1311075863f2f5ec

See also https://bugzilla.redhat.com/show_bug.cgi?id=1758263

Differential Revision: https://phabricator.kde.org/D24654