Default implementation for atk_util_[add/remove]_global_event_listener
Default implementation for atk_util_[add/remove]_global_event_listener

Implementing means that toolkits are not longer required to implement
those virtual methods, and is a first step to a multi-toolkit
environment.

Distrotech/atk,Distrotech/atk,Distrotech/atk,GNOME/at-spi2-core,GNOME/at-spi2-core,GNOME/at-spi2-core