When no theme is set, select sprites based on Boot$OSVersion
Detail: Prefer the Wimp$IconTheme first. If it is unset, pick sprites based on the UtilityModule's version from Morris4 or Ursula or Iyonix sets. Select some sprites even if the Boot application hasn't run (though it is still necessary to have "seen" !System to load critical modules from). Admin: Based on a suggestion from David Pilling.