dataengines/keystate: port away from dataengine

Task: https://phabricator.kde.org/T13329

Usage:

import QtQuick
import org.kde.plasma.private.keyboardindicator as KI

Text {
    text: capsLockState.locked ? "locked" : "unlocked"
    KI.KeyState {
        id: capsLockState
        key: Qt.Key_CapsLock
    }
}