gst/gstbin.c: Add helper function to find messages.

gst/gstbin.c: Add helper function to find messages.

Original commit message from CVS:
* gst/gstbin.c: (find_message), (bin_replace_message), (is_eos),
(gst_bin_remove_func), (gst_bin_element_set_state),
(bin_handle_async_start), (bin_handle_async_done),
(gst_bin_handle_message_func):
Add helper function to find messages.
Generate the async-done messages together with the state change
messages.
Small cleanups in handling toplevel bins.