Reinstate OCR bars and tooltips from prototype

[tomcrane]

Add in hook for additional manifest specific markup - in original sorting room, the OCR bars and tooltips with NER data. More generally the hook to inject arbitrary extra content per canvas.

[mattmcgrattan]

Consider, though, whether perhaps a single bar representing 'OCR quality' (a combination of the confidence and the spellcheck or something similar) might be better from a UI point of view.