Send and receive backup over network using QR code

This adds functionality to send and receive a backup over the network
using a QR code.

The sender or provider prepares the backup, sets up a server that
waits for clients.  It provides a ticket in the form of a QR code
which contains connection and authentication information.

The receiver uses the QR code to connect to the provider and fetches
backup, restoring it locally.

deltachat-ffi/src/lot.rs

@@ -52,6 +52,7 @@ impl Lot {
                 Qr::FprMismatch { .. } => None,
                 Qr::FprWithoutAddr { fingerprint, .. } => Some(fingerprint),
                 Qr::Account { domain } => Some(domain),
+                Qr::Backup { .. } => None,
                 Qr::WebrtcInstance { domain, .. } => Some(domain),
                 Qr::Addr { draft, .. } => draft.as_deref(),
                 Qr::Url { url } => Some(url),
@@ -103,6 +104,7 @@ impl Lot {
                 Qr::FprMismatch { .. } => LotState::QrFprMismatch,
                 Qr::FprWithoutAddr { .. } => LotState::QrFprWithoutAddr,
                 Qr::Account { .. } => LotState::QrAccount,
+                Qr::Backup { .. } => LotState::QrBackup,
                 Qr::WebrtcInstance { .. } => LotState::QrWebrtcInstance,
                 Qr::Addr { .. } => LotState::QrAddr,
                 Qr::Url { .. } => LotState::QrUrl,
@@ -127,6 +129,7 @@ impl Lot {
                 Qr::FprMismatch { contact_id } => contact_id.unwrap_or_default().to_u32(),
                 Qr::FprWithoutAddr { .. } => Default::default(),
                 Qr::Account { .. } => Default::default(),
+                Qr::Backup { .. } => Default::default(),
                 Qr::WebrtcInstance { .. } => Default::default(),
                 Qr::Addr { contact_id, .. } => contact_id.to_u32(),
                 Qr::Url { .. } => Default::default(),
@@ -175,6 +178,8 @@ pub enum LotState {
     /// text1=domain
     QrAccount = 250,
 
+    QrBackup = 251,
+
     /// text1=domain, text2=instance pattern
     QrWebrtcInstance = 260,