Log Message:
Update to 2.6.0

* Bug 635332: Include C header information in GIR
* Added atk_relation_set_contains_target to atk.symbols
* Bug 672869: Required a method to check if a relationset contains a
    relation taking into account relationship and target
* Added extra doc on atk_remove_[key/global]_event_listener
* Fixed compilation with some gcc versions/combination of options
* Provide a default implementation for
    atk_util_[add/remove]_global_event_listner
* Bug 478587: "accessible-role" - invalid default value
* Updated translations: Japanese, Brazilian Portuguese, Polish, Galician,
    Latvian, Catalan, Catalan (Valencian), Greek, Norwegian, Spanish