No event bus used for Ctrl+S

[Dreamsorcerer]

This is a minor niggle, but it seems like there is a good way to run code whenever the user downloads or prints, by using PDFViewerApplication.eventBus.on("download", ..) and PDFViewerApplication.eventBus.on("beforeprint", ..). This works if the user clicks the download or print button, and works with Ctrl+P for print.

However, Ctrl+S to trigger the download does not use the eventBus. It would be nice to also use the eventBus here, so there is a single point to hook onto.