Port to setAutoSaveSettings so that saving happens before hiding.

Summary:
See d35a882895 in kxmlgui for more complete explanation.

BUG: 396339
CCBUG: 395988

Test Plan: kmail ; Alt+F4 ; kmail -> now the statusbar and toolbar are visible again

Reviewers: ngraham, elvisangelaccio, broulik, cfeck, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14454