Use QStyleOption::initFrom() instead of init()

Avalaible since Qt 4.1.

Task-number: QTCREATORBUG-24098
Change-Id: I0dda1d78fc75070b6cbbef76d66bc54ccfca8f09
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/fancymainwindow.cpp

@@ -136,7 +136,7 @@ void DockWidgetTitleButton::paintEvent(QPaintEvent *)
     QPainter p(this);
 
     QStyleOptionToolButton opt;
-    opt.init(this);
+    opt.initFrom(this);
     opt.state |= QStyle::State_AutoRaise;
     opt.icon = icon();
     opt.subControls = {};