Fix private method registration to work

This makes <custom-a> display the correct URL for its properties
Make the property editor better in various ways

- When you blur from a property text field, it will update the field's value. This is important for e.g. CustomHTMLAnchorElement.prototype.href, where setting it to "index.html" will give back "http://localhost:8000/index.html".
- Don't make editable text fields for read-only string properties. Example: CustomHTMLAnchorElement.prototype.origin.
- Re-evaluate all properties, not just the ones that had editable text fields, when changing a property value. This allows e.g. CustomHTMLAnchorElement.prototype.origin to update when editing protocol.