gst/elements/gsttee.c: Remove usage of gst_pad_proxy_fixate.

gst/elements/gsttee.c: Remove usage of gst_pad_proxy_fixate.

Original commit message from CVS:
* gst/elements/gsttee.c: (gst_tee_init), (gst_tee_request_new_pad):
Remove usage of gst_pad_proxy_fixate.
* gst/gstcaps.c: (gst_caps_append), (gst_caps_append_structure),
(gst_caps_split_one), (gst_caps_replace):
Add poisoning code.
* gst/gstmarshal.list:
Add pointer__pointer for fixate signal
* gst/gstpad.c: (gst_real_pad_class_init),
(_gst_real_pad_fixate_accumulator), (gst_pad_link_fixate),
(_gst_pad_default_fixate_func), (gst_pad_proxy_fixate),
(gst_pad_set_explicit_caps), (gst_pad_template_new):
Add poisoning code. Add fixate signal on RealPad. Change
set_explicit_caps() to take const GstCaps, like try_set_caps().
* gst/gstpad.h:
* testsuite/caps/Makefile.am:
* testsuite/caps/app_fixate.c: Add a test for the fixate signal