Store stashed model index as QPersistentModelIndex

Summary:
Otherwise we can crash if the model updates during load.

CCBUG: 390005
CCBUG: 385557

Test Plan:
Had a state where my manually saved session would crash systemsettings on load
accessing a null kservice object from a menuItem

With this patch that session no longer crashes

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24785