Add an information blurb option to PromptWindow.

- PromptWindow now takes a parameter which contains optional
  informational text to display above the text control.

- Adjust callers.
Fix broken ValueNodeChildrenDeleted() behavior.

- Due to the way the item list was being iterated,
  ValueNodeChildrenDeleted() would only actually remove every
  other child.
Add RangeList utility class.

- Tracks a set of ranges. Implicitly deals with collapsing overlapping
  ranges as they're added.
Remove unnecessary cast.
Add utility functions for parsing a range expression.

- Fills out the supplied RangeList with the set of ranges that
  the expression stipulates.
Implement setting visible ranges on containers.

- Objects that act as ranged containers now expose a menu option
  which allows the user to input a comma-separated list of ranges to
  show. These need not be contiguous, ergo one can show only
  the first and last elements of an array, or some random set of
  indices in the middle as well.
Implement ranged container hooks for B{Object}List handler.