KCMultiDialog: Fix crash when clicking OK

When one closes KCMultiDialog with OK, QDialog::finished() can be emitted
before the clicked() signal of the OK button. This causes the following
sequence to happen:

- KCMultiDialogPrivate::_k_dialogClosed
* deletes the KCModule
- KCModuleProxyPrivate::_k_moduleDestroyed
* sets kcm to 0

- KCMultiDialog::slotOkClicked
- KCMultiDialogPrivate::apply
- KCModuleProxy::save
- KCModuleProxyPrivate::realModule
* notices kcm is 0, so recreates it
* calls kcm->save()
- KWinDesktopConfig::save()
* crashes because it expects kcm->load() to have been called

To avoid this, trigger the cleanup code in closeEvent() rather than when
finished() is emitted, as we can be sure closeEvent() is always called
*after* the methods connected to the button box signals has executed.

REVIEW: 118639