Streamlime verification control

[RussKie]

• Rename TaskDialogCheckBox -> TaskDialogVerificationCheckBox
• Add an overload to set Checked via ctor

  var verificationText = new TaskDialogVerificationCheckBox("I know what I'm doing", checked: true);

• Add an implicit operator to reduce boilerplate code:

  page.Verification = new TaskDialogVerificationCheckBox("I know what I'm doing");

  to

  page.Verification = "I know what I'm doing";