bin: iterate_sorted: Ensure sources are always returned last

bin: iterate_sorted: Ensure sources are always returned last

For linked elements, the resulting gst_bin_iterate_sorted() will
properly return elements from sink to sources.

If we have some elements that are not linked, we *still* want to
ensure that we return:
* In priority any sinks
* Last of all any sources
* And in between any element which is neither source nor sink

For this to work, when looking for the next candidate element,
not only check the degree order, but if there are two candidates
with the same degree order, prefer the non-source one.

Amongst other things, this fixes the case where we activating a
bin containing unlinked sources and other elements. Without this
we could end up activating sources (which might start adding pads
to be linked) before other (to which those new source element pads
might be linked) are not activated

https://bugzilla.gnome.org/show_bug.cgi?id=788434