Allow the title of "Content item" widgets to be overridden

The "Content item" widget allows you to place a single node on another page. This can be a useful way of reusing content in a limited way!

However, there are a couple of use cases when you may want to set the title:

1. When showing fields from the node, you may want to give it some kind of title, like "Featured article"
2. Or when showing the full node, you may want to use a different title than the full node, maybe something shorter to fit on a home page

The first use case is simple - we just enable title overrides on the View. The second is a little more complicated because it's actually rendering the node, so we need to inject the overridden title into the node at the correct point.

I'll upload a patch in a moment which does this!