gmain: add g_source_add_child_source and g_source_remove_child_source

gmain: add g_source_add_child_source and g_source_remove_child_source

This adds "child source" support to GSource. A child source behaves
basically like a GPollFD; when you add a source to a context, all of
its child sources are added with the same priority; when you destroy a
source, all of its child sources are destroyed; and when a child
source triggers, its parent source's dispatch function is run.

Use cases include:

    - adding a GTimeoutSource to another source to cause the source to
      automatically trigger after a certain timeout.

    - wrapping an existing source type with a new type that has
      a different callback signature

    - creating a source that triggers based on different conditions
      at different times.

https://bugzilla.gnome.org/show_bug.cgi?id=634239