Support for pageBreakSource property

[mattgarrish]

We added a new pageBreakSource property for identifying the pagination source in EPUBs but Ace does not recognize it and emits a serious error about the source not being specified when there are page breaks.

This property replaces dc:source + source-of, although both will have to be allowed. We should prefer pageBreakSource going forward, but not sure if we want to work that into an Ace "other" message.

[danielweck]

a11y:pageBreakSource vs pageBreakSource, see inconsistent specification examples:

https://www.w3.org/publishing/a11y/page-source-id/#examples

https://www.w3.org/TR/epub-a11y-tech-11/#pageSource

[mattgarrish]

The property should not have a prefix. It is going in the meta property vocabulary to replace the source-of property.

I'll file an issue to fix the techniques.