Whether the task has completed, meaning its callback (if set) has been
invoked. This can only happen after g_task_return_pointer(),
g_task_return_error() or one of the other return functions have been called
on the task.
This property is guaranteed to change from
TRUE exactly once.
GObject::notify signal for this change is emitted in the same main
context as the task’s callback, immediately after that callback is invoked.