Widget Engine Domain Access compatibility with Domain 8.x-1.0-beta8

Problem/Motivation

Widget Engine Domain Access compatibility with Domain 8.x-1.0-beta8

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Error: Undefined constant "DOMAIN_ACCESS_FIELD" in widget_engine_domain_access_views_data_alter() (line 292 of modules/contrib/widget_engine/modules/widget_engine_domain_access/widget_engine_domain_access.module).

Steps to reproduce

Enable "Widget Engine Domain Access" module.

Proposed resolution

Several PHP constants from Domain Access module (like DOMAIN_ACCESS_FIELD) have been moved to an interface. Need to update all the occurrences.