Create a better seam in PrimaryWindow to negotiate which content to display

[mejackreed]

Inspired in https://github.com/WilsonAndrewWilson/mirador-model-viewer/pull/1 . You may override the PrimaryWindow.prototype.renderViewer method, but will apply to all windows and then you must re-evaluate all of the Mirador logic.

We need some type of prop that a plugin can pass in that is a processor chain.