Fix crash during shutdown

Summary:
Fix crash during shutdown. The root cause is that when Dolphin in stopped as
part of an activity, the KItemListViewAccessible destructor is called after
QApplication::exec has returned causing Qt to already having cleaned up the
QAccessibleInterface instances kept in KItemListViewAccessible. Instead of
storing the pointers to QAccessibleInterface we store the QAccessible::Id so
that we can use the QAccessible::deleteAccessibleInterface function for
deleting the instances.

BUG: 402784

Test Plan:
I wasn't able to reproduce the crash in the first place, but I have just
opened and closed Dolphin a few times and verified the the QAccessibleInterface
instances are correctly cleaned up.

Reviewers: #dolphin, elvisangelaccio, ngraham

Reviewed By: #dolphin, elvisangelaccio

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19083