Confirmation Box text content format

And XWiki.widgets.ConfirmationBox.textContentFormat() method is now available. It indicates how the text parameter is interpreted.
Callers can check for the presence of this method to decide whether to escape their input:

• When this method is available, the text parameter is treated as plain text and must not be escaped.
• When this method is missing (before 18.4.0RC1/17.10.9), the text parameter is interpreted as HTML and may need to be escaped.
  The returned value is always the "plain" string.