Activity.get API - Filter by target_contact_id

(This should be written as a patch for CiviCRM 4.4. It's an offshoot of HR-238.)

As a bit of background, note the main call-path for Activity.get API is:

civicrm_api3_activity_get(...)
=> _civicrm_api3_basic_get(...)
=> _civicrm_api3_dao_set_filter(...)

For implementing the target_contact_id filter, one idea discussed tonight with Eileen on IRC:

• Modify _civicrm_api3_dao_set_filter() to call $dao->setApiFilter($params)
• Add CRM_Core_DAO::setApiFilter($params) – which does nothing
• Add CRM_Activity_BAO_Activity:setApiFilter($params) – which checks for $params['target_contact_id'] and calls $this->joinAdd() and $this->whereAdd()
• Add unit-tests to core for cases with (a) no matching activities, (b) a matching activity where the target_contact_id is the only target contact, (c) a matching activity with additional (irrelevant) target contacts.
• Make sure the unit-test api_v3_ActivityGetAbsencesTest::testGetTarget() passes