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-jsonrpc/src/api/types/qr.rs

@@ -32,6 +32,9 @@ pub enum QrObject {
     Account {
         domain: String,
     },
+    Backup {
+        ticket: Vec<u8>,
+    },
     WebrtcInstance {
         domain: String,
         instance_pattern: String,
@@ -126,6 +129,9 @@ impl From<Qr> for QrObject {
             }
             Qr::FprWithoutAddr { fingerprint } => QrObject::FprWithoutAddr { fingerprint },
             Qr::Account { domain } => QrObject::Account { domain },
+            Qr::Backup { ticket } => QrObject::Backup {
+                ticket: ticket.to_bytes(),
+            },
             Qr::WebrtcInstance {
                 domain,
                 instance_pattern,