backport commit b72fc5e56579035bf987075e16324ef95ef8e3d4

Author: Alex Merry <alex.merry@kde.org>
Date:   Mon Oct 19 20:54:13 2015 +0100

    Use deleteLater in Part::slotWidgetDestroyed().

    When a part's main widget is destroyed, we should not delete the part
    immediately - both because deleting an object from inside one of its
    signals is not safe generally, and because derived classes may well hold
    QPointers to the widgets, expecting them to be cleared between the
    widget being destroyed and the part being destroyed.

MERGE: none