Backup improvements & example

XXMessengerClient library

• Fix race condition issues when starting the backup and setting initial parameters.
• Add BackupStorage utility that can store the backup.
• Store BackupStorage.Backup on disk.

xx-messenger example

• Start backup
• Resume backup
• Stop backup
• Store backups in storage
• Observe backup storage
• Export backup from storage
• Resume backup on launch

Examples/xx-messenger/Sources/BackupFeature/BackupView.swift

@@ -43,12 +43,12 @@ public struct BackupView: View {
     WithViewStore(store, observe: ViewState.init) { viewStore in
       Form {
         Group {
-          if !viewStore.isRunning {
-            newBackupSection(viewStore)
-          }
           if viewStore.isRunning || viewStore.backup != nil {
             backupSection(viewStore)
           }
+          if !viewStore.isRunning {
+            newBackupSection(viewStore)
+          }
         }
         .disabled(viewStore.isLoading)
         .alert(