Add types to represent identifiers for frame child lists, sets of those
identifiers, and iterators over the child lists of a frame.
Define a enumeration type FrameChildListID representing all of the types
of child lists that frames have (each with a unique bit), a class
FrameChildListIDs for representing any set of FrameChildListID, and a
class FrameChildListIterator for iterating over all of the child lists
of a frame.
