Note: Do not change the body if it hasn't been modified

I'm scared that for very large notes this won't be very fast, on the
other hand changing the body results in many other fields being
re-computed.

lib/core/note.dart

@@ -109,6 +109,10 @@ class Note with NotesNotifier {
   }
 
   set body(String newBody) {
+    if (newBody == _body) {
+      return;
+    }
+
     _body = newBody;
     _summary = null;
     _links = null;