QQuickScreen.name can change at runtime

It's generally a constant but the attached property doesn't always
have its QScreen at startup, so in practice every property needs a
notifier.  (Followup to Id777caf6cef1df5bc01757605e3085151170760b )

Change-Id: Ia3a321f98dc49b7e75f6eceb8f16b153c73a3134
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>