CRM_Core_BAO_MessageTemplates::add() should call pre/post hooks.

Currently CRM_Core_BAO_MessageTemplates::add() doesn't call the CRM_Utils_Hook::pre() and CRM_Utils_Hook::post() hooks like many other object such as CRM_Mailing_BAO_Mailing. Adding an invocation of these hooks would lessen the need for integrators to override the source to introduce customizations.