Basic implementation of undo/redo

It's not great, as the granularity is too low. But it's a good first
step. It only works in the RawEditor so far.

lib/editors/checklist_editor.dart

@@ -185,6 +185,8 @@ class ChecklistEditorState extends State<ChecklistEditor>
       ),
       onUndoSelected: _undo,
       onRedoSelected: _redo,
+      undoAllowed: false,
+      redoAllowed: false,
     );
   }