RemoteImageWidget does not respect the configuration of the field

What started as a small adjustment in two fields of the form element, ended up as a rather large change in the code base.

In the following lines I describe the changes that the attached patch would make.

First of all, a schema.yml file was added. It contains the definitions for the field settings, the widget and the field value.

I use the schema.yml file of the links module as base, since the remote_image module extends the functionality of that field.

I also use the concepts of the links module for adding the validation of the subelements when the field is required.

A few practical examples of the validations:

* If the field is required and the alt is optional, the validation won't fail if the alt is empty.
* If the field is required and the alt is required, the validation will fail if the alt is empty.
* If the alt has a value, the URL will be required. This works the same way for the rest of the subelements.

Finally, I added a hook_update, which migrates the existing field settings to the new keys alt_attribute and title_attribute.