CiviCRM Settings can only be overloaded if they are scalars

A goal of the CRM_Core_BAO_Setting::getItem() approach to configuration is to allow downstream to specify settings as .php files which can be easily transferred between different CiviCRM instances. Specifically, it allows one to use statements like:

define('settinggroup.settingname', 'settingvalue');

This works if the value is a scalar. However, if the value is an array (like with 'mailing_backend'), then it fails.

Proposal: Update getItems() to also read settings from a global array, e.g.

$civicrm_setting['settinggroup']['settingname'] = 'settingvalue';

(I can fix this very quickly – but I wanted to make sure there wasn't some big design consideration.)

As an example use-case, it would be helpful to unit-test email applications by replacing the normal email handler (SMTP, sendmail, etc) with a mock (in-memory) handler. This could be done by allowing the unit-test to modify the 'mailing_backend' option (for the duration of the test).

(The patches here are very simple - I'm making a ticket to get a second-opinion on the design.)