gst/adder/: Ported adder as an example of a mixer element using collect pads. Needs more negotiation work.

Original commit message from CVS:
* gst/adder/Makefile.am:
* gst/adder/gstadder.c: (gst_adder_setcaps),
(gst_adder_class_init), (gst_adder_init),
(gst_adder_request_new_pad), (gst_adder_collected),
(gst_adder_change_state):
* gst/adder/gstadder.h:
Ported adder as an example of a mixer element using
collect pads. Needs more negotiation work.