Let the user sort fields. Make sure the Editor shows up correctly.

r? @ametaireau 
Here: https://github.com/spiral-project/formbuilder/pull/21/files#diff-5791846de8054ba5cc9a04ef8d5260acR31
And here: https://github.com/spiral-project/formbuilder/pull/21/files#diff-5791846de8054ba5cc9a04ef8d5260acR43

Also it will looks weird here: https://github.com/spiral-project/formbuilder/pull/21/files#diff-5791846de8054ba5cc9a04ef8d5260acR44

I'd rather like to use `element.id`