add support for <nasal> blocks <open> and <close> in XML dialogs:

add support for <nasal> blocks <open> and <close> in XML dialogs:

  <nasal>
      <open>print("I'm called on dialog open")</open>
      <close>print("I'm called on dialog close")</close>
  </nasal>

All Nasal runs in a dialog namespace, so that variables and functions
defined in the <open> block can be used in <binding>s, etc. This is
especially useful for <radio> button handling. See "location-in-air.xml".