Add threadsafe replacement functions for getting internal links of an element. Deprec...

Add threadsafe replacement functions for getting internal links of an element. Deprecate the old internal links funct...

Original commit message from CVS:
Based on patch by: Olivier Crete <tester at tester dot ca>
* docs/gst/gstreamer-sections.txt:
* win32/common/libgstreamer.def:
* gst/gstpad.c: (gst_pad_init),
(gst_pad_set_iterate_internal_links_function),
(int_link_iter_data_free), (iterate_pad),
(gst_pad_iterate_internal_links_default),
(gst_pad_iterate_internal_links), (gst_pad_get_internal_links):
* gst/gstpad.h:
Add threadsafe replacement functions for getting internal links of an
element. Deprecate the old internal links functions.
API:GstPad::gst_pad_set_iterate_internal_links_function()
API:GstPad::GstPadIterIntLinkFunction
API:GstPad::gst_pad_iterate_internal_links()
API:GstPad::gst_pad_iterate_internal_links_default()
* gst/gstghostpad.c: (gst_proxy_pad_do_iterate_internal_links),
(gst_proxy_pad_init):
Implement threadsafe internal links.
* tests/check/elements/tee.c: (GST_START_TEST), (tee_suite):
Unit test for internal links on tee. See #549504.