gst/gstbin.*: Add do-latency signal with the old default fallback implementation. This allows for custom latency calc...

Original commit message from CVS:
* gst/gstbin.c: (_gst_boolean_accumulator), (gst_bin_class_init),
(gst_bin_recalculate_latency), (gst_bin_do_latency_func),
(gst_bin_change_state_func):
* gst/gstbin.h:
Add do-latency signal with the old default fallback implementation. This
allows for custom latency calculations for when the default is not
sufficient.
API: GstBin::do-latency signal.