Revert unpopular change for GST_MESSAGE_SRC to GObject.

Revert unpopular change for GST_MESSAGE_SRC to GObject.

Original commit message from CVS:
* check/gst/gstbin.c: (GST_START_TEST):
* gst/gstbin.c: (bin_bus_handler):
* gst/gstmessage.c: (gst_message_finalize), (_gst_message_copy),
(gst_message_new), (gst_message_new_eos), (gst_message_new_error),
(gst_message_new_warning), (gst_message_new_tag),
(gst_message_new_state_changed), (gst_message_new_segment_start),
(gst_message_new_segment_done), (gst_message_new_custom):
* gst/gstmessage.h:
* tools/gst-launch.c: (event_loop):
* tools/gst-md5sum.c: (event_loop):
Revert unpopular change for GST_MESSAGE_SRC to GObject.