(dev/core#1387) ConfigSetting - Deprecation+guard for retrieve()/add()

(dev/core#1387) ConfigSetting - Deprecation+guard for retrieve()/add()

These functions were traditionally used for accessing `config_backend`,
which was (mostly) dropped in v4.7 and became inert. However, the
functions are still referenced in a couple oddball cases.

Note that the references to `$domain->config_backend` are liable to raise
warnings if evaluated - since the field no longer exists on the DAO class.
Consequently, the extra `$hasBackend` guard should reduce warnings.