Display definition title when content item has no title

When editing a content item with a null or empty string `Title` property, the editor currently displays the header:
> Edit ""

Let's fall back to the title from the definition instead, similar to how the header is rendered when adding a new content item. Example:
> Edit "Analytics Tracking Code"

A similar change is provided for the `ItemLink` control, allowing fall back to definition title in the right hand side listing of children in zones.