Minimal history handler, handle node move. Auto parse JSON content for accurate typing.

types/content.d.ts

@@ -15,7 +15,7 @@ export interface Overview {
 }
 export interface CanvasContent extends Overview {
     type: 'canvas';
-    content?: string;
+    content?: Canvas;
 }
 export interface MapContent extends Overview {
     type: 'map';