Fix conflict between QMainWindow::restoreState() and QWidget::setStylesheet()

Fix conflict between QMainWindow::restoreState() and QWidget::setStylesheet()

If QMainWindow::restoreState() then QWidget::setStylesheet() were called
before the QMainWindow is shown, the size of QDockWidget can not be
restored.

QWidget::setStylesheet() will generate QEvent::StyleChange event, which will
cause the function QDockAreaLayout::fitLayout() to be called before the layout
of MainWindow is activated. Although the state info has been stored in
a QMainWindowLayoutState variable by QMainWindow::restoreState(), but
QMainWindowLayout::setGeometry() still isn't called at present. So
QDockAreaLayout::fitLayout() will force the size of dockwidgets
and centralwidget to be calculated using the wrong geometry, which will
break the state restored by QMainWindow::restoreState().

This is a side effect of 692e9103ebb85b90e79377206d5d03b704d43d42.

Task-number: QTBUG-15080

Change-Id: I8cda6a529d178f7467a59b780db80df0a44d4769
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>