Interface for the JUndo class.
Base class to support the fundamental concept of undoing an operation.
We require that all derived classes implement Undo().
We maintain an "active" flag because some derived classes will accumulate
a set of changes into one undo action. A deactivated Undo object can no
longer accumulate changes and must therefore be replaced before the next
change.
We also maintain a "redo" flag so other objects can tell whether we
are actually performing an undo or a redo operation. (Redo objects should
only be created by Undo objects.)
