[LocalBackup] Implement missing backup types in the file (backend)

[jrainville]

Following the POC found here, we need to finish the implementation.

What's missing:

• settings backup
• keypairs (do we really want those? I think we decided to no longer backup those?)
• watch only accounts
• add tests for all the backup type (can be in the same test, but all types must be tested)

[jrainville]

keypairs (do we really want those? I think we decided to no longer backup those?)

@alaibe can you tell me if the keypairs in the backup is the thing we decided we no longer needed to backup because it's useless, since you have to re-import using a private key or mnemonic anyway?