gst/gstbin.c: Improve debugging.

gst/gstbin.c: Improve debugging.

Original commit message from CVS:
* gst/gstbin.c: (is_eos), (gst_bin_add_func),
(bin_handle_async_start), (gst_bin_handle_message_func):
Improve debugging.
When adding elements, insert messages into the bus of the newly added
element and make sure the element is the source of the message. This
allows the parent bin to intercept the message and do the
right thing. It also avoids us posting ASYNC_START and CLOCK_PROVIDE
messages to the app (which is not allowed).
Update some docs.
* tests/check/gst/gstghostpad.c: (GST_START_TEST):
Fix testsuite so that is does not work around messages that should not
have been posted in the first place.