Replace low-level keystore backend crates with sequoia-keystore
In !493 (merged) we've replaced our custom key store implementation with sequoia-keystore-softkeys
crate. This migration didn't require any changes on the user UI.
However, we should migrate to the high-level sequoia-keystore
crate, which offers additional features and requires less code on the sett
side. This migration requires a change on the UI: adding an option for deleting keys (instead of displaying a path to the file that store the key).
Proposed by @nwalfield