Rewrite e_load_book_source_async().

Rewrite e_load_book_source_async().

This is a late libedataserverui API break, but so be it.

Rewrite e_load_book_source_async() to use a GIO-style asynchronous
pattern, and add a corresponding e_load_book_source_finish().

The API is fairly obvious:

   void     e_load_book_source_async   (ESource *source,
                                        GtkWindow *parent,
                                        GCancellable *cancellable,
                                        GAsyncReadyCallback callback,
                                        gpointer user_data);

   EBook *  e_load_book_source_finish  (ESource *source,
                                        GAsyncResult *result,
                                        GError **error);

This also eliminates the thread for loading EBooks from ENameSelector.