gdp: add gst_object_{s,g}et_control_rate(), add private data section, fix docs

gdp: add gst_object_{s,g}et_control_rate(), add private data section, fix docs

Original commit message from CVS:
* docs/libs/gstreamer-libs-sections.txt:
* libs/gst/controller/gstcontroller.c:
(_gst_controller_get_property), (_gst_controller_set_property),
(_gst_controller_init), (_gst_controller_class_init):
* libs/gst/controller/gstcontroller.h:
* libs/gst/controller/gsthelper.c: (gst_object_get_control_rate),
(gst_object_set_control_rate):
API: add gst_object_{s,g}et_control_rate(), add private data section,
fix docs
* libs/gst/dataprotocol/dataprotocol.c: (gst_dp_packetizer_new):
* libs/gst/dataprotocol/dataprotocol.h:
add deprecation guards to make gtk-doc happy and allow disabling cruft