winui: make SecurePasswordBox notify all changes

Update the `SecurePasswordBox` control to notify all listeners of
changes to the password value. Previously we only notified on the first
character typed into the box.

We clone the current password, modify it, and then replace the previous
instance (triggering an `INotifyPropertyChanged::PropertyChanged`
event). With a binding delay, we will batch updates of multiple
keystrokes rather than one at a time.

Author: mjcheetham

src/windows/Shared.UI.Windows/Controls/SecurePasswordBox.cs

@@ -47,6 +47,7 @@ public SecureString Password
         }
 
         private bool updatingTextFromPasswordChange;
+
         private void UpdateText()
         {
             updatingTextFromPasswordChange = true;
@@ -69,14 +70,17 @@ protected override void OnTextChanged(TextChangedEventArgs e)
         {
             if (!updatingTextFromPasswordChange)
             {
-                SecureString password = Password;
+                // Update a copy of the password SecureString so that we can trigger a changed event
+                // for anyone listening on the actual password property.
+                SecureString password = Password.Copy();
                 int selectionStart = SelectionStart;
                 foreach (TextChange change in e.Changes)
                 {
                     password.RemoveAt(change.Offset, change.RemovedLength);
                     password.InsertAt(change.Offset, Text.Substring(change.Offset, change.AddedLength));
                 }
                 UpdateText();
+                Password = password;
                 SelectionStart = selectionStart;
             }
             base.OnTextChanged(e);