VeraCrypt keyfile widget in GNOME Disks is unreachable with the keyboard

Noticed while writing the automated tests. Apparently it’s not on the list of widgets one can select with “TAB”. Also, it has no description (in accerciser speak, probably translates to tooltip in GTK). Both combined make it very hard to interact with programmatically, and probably impossible with at least some kinds of universal access technologies.

MR: https://gitlab.gnome.org/GNOME/gnome-disk-utility/merge_requests/24

Parent Task: #14468

Original created by @intrigeri on 15952 (Redmine)

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