Leaf.Core v1.3.17

Move namespace Leaf.Core.Extensions.System to Leaf.Core.Extensions.

New methods: ThreadSafeUI.IsCanceledByUser(Exception).
New extensions:
SmtpClient.SendMailAsync(MailMessage, CancellationToken);
string.IsNullOrEmpty();
string.HasContent();
Task.Caf() = Task.ConfigureAwait(false);