Part of the rotation handling code, in what's now `web/app.js`, hasn't really changed since before the viewer was split into multiple files/components.
Similar to other properties, such as current page/scale, we should probably avoid tracking state in multiple places. Hence I'm suggesting that we don't store the rotation in `PDFViewerApplication`, and access the value in `PDFViewer` instead.
Since `PDFViewerApplication.pageRotation` has existed for a very long time, a getter was added to avoid outright breaking third-party code that may depend on it.