[dolphinview] Disconnect modelChanged when destructing

Deleting DolphinView deletes KItemListContainer, which deletes
KItemListController, which emits modelChanged, which causes a
signal delivery to the already destroyed DolphinView

This is never good, and in Qt6 causes an assert
(https://codereview.qt-project.org/c/qt/qtbase/+/381860

).

Co-authored-by: Nicolas Fella <nicolas.fella@gmx.de>