Fix title validation

This MR fixes the validation of the Title Object which has formerly not been working very well.

The schema adjustments ensure that:

• if several Title Objects are available in an array, there is one and only one main title
• if no subtitle is given, there is only one Title Object present (which represents a main title)

It does so by leveraging the prefixItems property which as a consequence requires implementers to put the main title on array position 0.

Furthermore, this MR adds a missing requires @context to title-main.json.