documents: Use recommended app as the external viewer

This avoids ZIP-based documents from being opened with the Archive
Manager. This is especially important for ePubs and CBZ files.

https://bugzilla.gnome.org/show_bug.cgi?id=740978
documents: Don't open docs without a recommended app

When a document type doesn't have a recommended application (or no
external application at all), don't try to force open it.

This prevents selected but unhandled files from being opened if one
of the selected files can be opened (eg. one ePub + one PDF should only
open the PDF file, and ePub shouldn't get opened in file-roller).

https://bugzilla.gnome.org/show_bug.cgi?id=740978