ItemDelegate label always uses mnemonics

The various ItemDelegate/MenuItem/Button implementations internally use QQuickMnemonicLabel to render the text property. While this is ok for Menus and Buttons this causes problems for other use cases when an ItemDelegate is used for showing model data with a ListView (which is the ItemDelegate use case after all).

The ComboBox implementation also shows this problem, just open the example's combobox popup.

AbstractButton should have a property that controls whether the default contentItem uses mnemonics or not (defaulting to true for actual button implementations and to false for ItemDelegate etc).

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12

Window {
  visible: true
  width: 640
  height: 480
  title: qsTr("Hello World")

  ListView {
    anchors.fill: parent
    model: ["This & That", "Cat & Dog"]
    delegate: ItemDelegate {
      text: modelData
    }
  }

  ComboBox {
    anchors.centerIn: parent
    model: ["This & That", "Cat & Dog"]
  }
}