Add background for modal dialog
Use the same effect as in Kirigami.Dialog. See https://invent.kde.org/frameworks/kirigami/-/blob/master/src/controls/Dialog.qml?ref_type=heads#L308
Use the same effect as in Kirigami.Dialog. See https://invent.kde.org/frameworks/kirigami/-/blob/master/src/controls/Dialog.qml?ref_type=heads#L308