Qt 5.8.0 introduce new Qt.labs.platform module with Platform Menu implementation.
I've try create context menu with sub menu like:
On Linux (Debian) all work fine, but there are no sub menus on Windows.
Looking to the code I've found QWidget backend is used on Windows. But QWidget backend don't implement sub menus.
I use QtQuick 2 only and has no lot of expierence in QtWidgets, but seems it should work (see Qt Widgets Menu Example)