Render (optional) transcriptions in transparent layer for text selection

[tomcrane]

Painting the text into an invisible but selectable layer on top of the canvas (as you can do in a PDF), so that users can magically cut and paste text out of a IIIF resource.

CanvasPanel can support multiple modes of rendering textual content - somewhere else, in cooperation with the containing app, or directly, with various degrees of visibility.

Related to #176