element: add gst_element_foreach_*pad()

element: add gst_element_foreach_*pad()

Add convenience API that iterates over all pads, sink pads or
source pads and makes sure that the foreach function is called
exactly once for each pad.

This is a KISS implementation. It doesn't use GstIterator and
doesn't try to do clever things like resync if pads are added
or removed while the function is executing. We can still do that
in future if we think it's needed, but in practice it will
likely make absolutely no difference whatsoever, since these
things will have to be handled properly elsewhere by the element
anyway if they're important.

After all, it's always possible that a pad is added or removed
just after the iterator finishes iterating, but before the
function returns.

This is also a replacement for gst_aggregator_iterate_sink_pads().

https://bugzilla.gnome.org/show_bug.cgi?id=785679