public void add_child_source (Source child_source)
child_source to this as a "polled" source; when this is
added to a MainContext,
child_source will be automatically added with
the same priority, when
child_source is triggered, it will cause this to dispatch (in addition to
calling its own callback), and when this is destroyed, it will destroy
child_source as well.
(this will also still be dispatched if its own prepare/check functions indicate that it is ready.)
If you don't need
child_source to do anything on its own when it triggers, you can call
source_set_dummy_callback on it to set a callback that does
nothing (except return true if appropriate).
this will hold a reference on
child_source is attached to it.
a second SignalSource that this should "poll"