Use correct window for getting ComboBox current icon pixmap

In widgets:
* windowHandle() only works for top level widgets (windows), so we need to first get the actual window() as windowHandle() is always
  nullptr for controls like ComboBox

In QtQuick Controls:
* we would just blow up for accessing widget which is null here. Use the QQuickWindow of the QQuickItem, if any. Given how long this
  went unnoticed shows that Qt *never* ever had icons working in ComboBoxes in QtQuick...

Differential Revision: https://phabricator.kde.org/D18655