Fix the "clearing passwords in 45s" function

- Copied passwords in the extension are not cleaned.

pass/Helpers/SecurePasteboard.swift

@@ -0,0 +1,55 @@
+//
+//  SecurePasteboard.swift
+//  pass
+//
+//  Created by Yishi Lin on 2017/7/27.
+//  Copyright © 2017 Bob Sun. All rights reserved.
+//
+
+import Foundation
+import UIKit
+
+class SecurePasteboard {
+    public static let shared = SecurePasteboard()
+    private var backgroundTaskID: UIBackgroundTaskIdentifier? = nil
+    
+    func copy(textToCopy: String?, expirationTime: Double = 45) {
+        // copy to the pasteboard
+        UIPasteboard.general.string = textToCopy ?? ""
+        
+        // clean the pasteboard after expirationTime
+        guard expirationTime > 0 else {
+            return
+        }
+        
+        // exit the existing background task, if any
+        if let backgroundTaskID = backgroundTaskID {
+            UIApplication.shared.endBackgroundTask(backgroundTaskID)
+            self.backgroundTaskID = nil
+        }
+        
+        backgroundTaskID = UIApplication.shared.beginBackgroundTask(expirationHandler: { [weak self] in
+            guard let taskID = self?.backgroundTaskID else {
+                return
+            }
+            if textToCopy == UIPasteboard.general.string {
+                UIPasteboard.general.string = ""
+            }
+            UIApplication.shared.endBackgroundTask(taskID)
+        })
+        
+        DispatchQueue.global(qos: .utility).asyncAfter(deadline: .now() + expirationTime) { [weak self] in
+            guard let strongSelf = self else {
+                return
+            }
+            if textToCopy == UIPasteboard.general.string {
+                UIPasteboard.general.string = ""
+            }
+            if let backgroundTaskID = strongSelf.backgroundTaskID {
+                UIApplication.shared.endBackgroundTask(backgroundTaskID)
+                strongSelf.backgroundTaskID = nil
+            }
+        }
+    }
+    
+}