File contents, but also file sizes, metadata and directory structures should be kept confidential in elementary OS if required

## Prerequisites
- [x] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**

This issue is a follow-up to https://github.com/elementary/files/issues/1046

Inspired by the system-integrated KDE solution "Plasma Vault", I want to suggest such a solution for elementary OS. It is about inviting users to protect their personal spaces and data by means of privacy by design solutions. This idea can be seen as a useful addition to the full disk encryption, which elementary will introduce by default with the new installer.

https://www.omgubuntu.co.uk/2017/06/plasma-vault-create-encrypted-folder-linux
https://github.com/KDE/plasma-vault

"Plasma Vault" uses CryFS, a relatively new solution, but one that is very hopeful and actively being developed.

https://www.cryfs.org/comparison/
https://www.cryfs.org/howitworks/
https://nuetzlich.net/gocryptfs/comparison/

A graphical interface elementary OS should allow its users the following functions:

1. a way to display configured volumes.
2. an ability to display unlocked volumes.
3. an ability to view locked volumes.
4. an ability to unlock volumes.
5. an ability to lock volumes.

Volume unlocking and locking can optionally be enabled very well through support for Open Smart Card solutions.

I would like to present my idea for discussion and I am looking forward to the thoughts and designs from the elementary development team and the community!

**Describe the solution you'd like**
The solution is not intended purely as a local solution. Therefore it makes sense to include the Switchboard Online Accounts Plug from the beginning.

**Existing work**
So far, this has already been implemented at KDE by "Plasma Vault" as a system solution.
SiriKali https://mhogomchungu.github.io/sirikali/ is available as an app solution.