Disabled widget addition

I've added a widget that shows a disabled text_field with the value.

When creating a new entity the probable value is shown in the field.
The value shown might be wrong because another entity might have been created in the meanwhile, added to the #description.
The 'next value' is retrieved querying the information_schema.
Database connection is injected

I've adjusted some comments to be more reasoning.

I'm not an expert on the testing part, but I've tried to do atleast as much testing as there was for the default widget. I've also replaced the deprecated functions from the test class.