Also display screenshots for development changes when they're defined

Right now the various Displayers and the Changes Sheet only display screenshots when the audience is set to user or admin. There's no reason to not have this feature for developer audience too.

Note that originally the Grid Displayer (which was the first displayer) was displaying all changes (user+admin and developer changes) and thus had some special handling to generate some combined view. However now the {{releasenotechanges}} macro decides how to display each audience so the need is no longer there.