Enable required keyword for alias properies

It would be nice to be able to mark an alias property as required:

// MyComponent.qml
Item {
  required property alias displayText: textBox.text
  Text {
    id: textBox
  }
}

An alternative would be to mark the inner property required. But having it on the alias allows us to:

• Only require it for the aliased property
• Have the API documented at the top of the component, so that users don't need to also look at the aliased property to know that it is required