Issue #2187829 by longwave, IshaDakota, ianthomas_uk, richard.c.allen2386: Convert all calls to check_plain() in core to Drupal\Component\Utility\String::checkPlain() in core/modules/views.

Issue #1908194 by  richard.c.allen2386: Document what happens in drupal_get_path if the file is not found

Issue #1899460 by chrisjlee, richard.c.allen2386: Replace duplicate defgroup tags with addtogroup in documentation blocks