shellMountOperation: Support TCRYPT

shellMountOperation: Support TCRYPT

This extends the ShellMountPasswordDialog by widgets which allow
specifying parameters supported by TrueCrypt and VeraCrypt compatible
volumes (TCRYPT). This includes:

 - Whether the volume to be unlocked is hidden.
 - Whether the volume to be unlocked is a system partition.
   Note: TrueCrypt and VeraCrypt only support encrypting Windows
   systems [1], so the label for this option is "Windows System Volume".
 - Whether to use a PIM [2].
 - Whether to use keyfiles. Unfortunately, GMountOperation doesn't
   support TCRYPT keyfiles, so if this checkbox is checked, we tell the
   user that they should unlock the volume with Disks, which supports
   unlocking TCRYPT volumes with keyfiles.

[1] https://www.veracrypt.fr/en/System%20Encryption.html
[2] https://www.veracrypt.fr/en/Header%20Key%20Derivation.html

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/126