gst/: Use gst_data_copy, instead of ref, since some applications that uses GBoxed routines depends on a function that...

Original commit message from CVS:
* gst/gstbuffer.c (_gst_buffer_initialize):
* gst/gstdata.c (gst_data_get_type):
* gst/gstevent.c (_gst_event_initialize): Use gst_data_copy,
instead of ref, since some applications that uses GBoxed
routines depends on a function that actually returns a copy.