parsebin: Improve elementary stream handling

parsebin: Improve elementary stream handling

The goal of parsebin is to figure out which elements to link together in order
to provide elementary streams given any random input.

The problem is that deciding whether a given stream should still have more
elements plugged in or not was dependent on ... the presence of compatible
decoders (sic).

Instead of that, if we can't plug anymore elements on a given stream *and* it is
detected as being an elementary stream, expose it.

Fixes #2118

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4237>