Adding the name of the asset being saved to the dialogue box

At the moment when you save an asset, it just says "Content changed. Do you want to save it?" But it doesn't give the name of the asset in question. This can become very confusing.

1. At the moment there is no name on the top of editor/json widget, and it is not obvious what is being changed. see #229 (closed) .
2. If there will be multiple windows in the future this feature is necessary. see #228 .