This calls a function from another thread, asynchronously. This is to be
used for cases when a state change has to be performed from a streaming
thread, directly via gst_element_set_state() or indirectly e.g. via SEEK
events.

Calling those functions directly from the streaming thread will cause
deadlocks in many situations, as they might involve waiting for the
streaming thread to shut down from this very streaming thread.

This is mostly a convenience function around a GThreadPool and is for example
used by GstBin to continue asynchronous state changes.

https://bugzilla.gnome.org/show_bug.cgi?id=760532