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.

src/context.rs

@@ -513,6 +513,13 @@ impl Context {
 
     // Ongoing process allocation/free/check
 
+    /// Tries to acquire the global UI "ongoing" mutex.
+    ///
+    /// This is for modal operations during which no other user actions are allowed.  Only
+    /// one such operation is allowed at any given time.
+    ///
+    /// The return value is a cancel token, which will release the ongoing mutex when
+    /// dropped.
     pub(crate) async fn alloc_ongoing(&self) -> Result<Receiver<()>> {
         let mut s = self.running_state.write().await;
         ensure!(