Issue #117: Provide Backdrop-unique constants for identifying Backdrop version.
Issue #117: Updating Backdrop versions in newly added modules.
Issue #117: Fixing tests affected by new Backdrop version constants.
Issue #117: Updating identifiers to use 'Backdrop CMS' for clarity.
Merge pull request #216 from quicksketch/117/backdrop-version

Issue #117: Identify Backdrop version and distinguish it from Drupal 8.x.