Streamline debug output of QMainWindowLayout.

Remove define Q_DEBUG_MAINWINDOW_LAYOUT and dependent exported
function, introduce operators for QDebug.

Task-number: QTBUG-38761
Change-Id: I7794cffbfaea20ead0eac9e8ace4e3a910c50f95
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
QMdiArea: Store focus widget when new QMdiSubWindow is added.

Introduce function QMdiSubWindowPrivate::storeFocusWidget()
to store focus widget and call this when de-activating a
sub window. Change restoreFocus() to return a bool and call
it from QMdiSubWindowPrivate::setActive().

Task-number: QTBUG-38378
Change-Id: I18dbe66ce85213ca5b4907b5a09126544415351a
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>