Updating plan by removing completed items
nodeEditHappened overridden in LurchEditor

All old tests pass but no new tests written for it yet.
The writing of such tests is added as a to-do in this commit.
Implemented canUndo/canRedo in LurchEditor

Added to-dos to project plan for creating corresponding unit tests, but
none have yet been created.
All old tests still pass.
Shortening undoRedoStackPointer to stackPointer
Implemented undoDescription/redoDescription

All old tests pass, no new ones written.
The writing of new tests added as to-dos in project plan.
Implemented undo/redo in LurchEditor

All old tests pass, no new tests written.
This commit adds to-dos for writing new unit tests.
Update plan re: loading/saving/sharing online/offline
Moved undo/redo stack tools into DOMEditTracker

They had been placed in LurchEditor (a subclass) but that made less
sense.
Added test for growing undo/redo stack & pointer
Added test for stack truncation, fixed bug
Added test for canUndo/canRedo
Added test for undo/redoDescription, fixed bugs
Added test for undo/redo in DOMEditTracker, fixed bugs