recent: Prevent spurious monitor notifications

Ignore nonexistent files completely to prevent them being repeatedly
deleted and then re-added to the list of files.

https://bugzilla.gnome.org/show_bug.cgi?id=686135
recent: Don't mark filesystem as read-only

The filesystem should not be marked as read-only since items can be
deleted from it.

https://bugzilla.gnome.org/show_bug.cgi?id=710993