Notify the user about form data loss when another document is opened

[timvandermeij]

Since #12241 we have notifications for this when the user navigates away from the viewer, but we do not yet warn when switching documents, which means that if you call e.g. PDFViewerApplication.open with a new document (or directly call BaseViewer.setDocument when using the viewer components standalone) there won't be any warning about losing form data. This should be addressed.

[Snuffleupagus]

Given that this obviously doesn't apply to the Firefox PDF Viewer, the idea of having to add a custom prompt (e.g. somewhat similar to the existing password prompt) and the associated logic doesn't feel all that great as far as I'm concerned.

Hence, taking a step back and considering that it's somewhat of an edge-case in the GENERIC viewer, considering that what we really want to avoid here is unexpected data loss in forms I'm wondering if we could perhaps "cheat" a bit here:
What if we simply triggered a normal save dialog automatically, from within the PDFViewerApplication.close method, when modified form data exists (assuming that'd actually work, given how the GENERIC DownloadManager works)?

[timvandermeij]

I think that would also be fine. The intention is indeed to avoid unexpected data loss, so prompting the user to save the form would also suffice I think.