encodebin: Fix and refactor smart encoding

encodebin: Fix and refactor smart encoding

It was not working properly and the implementation of the smartencoder
element was weird. This introduce a number of changes (which are all
in one single commit because they basically all work together and lead
to basically reimplementing the element):

* Make smartencoder a bin so that the reencoding chain of elements are
  inside of it instead of not having any parent. Those elements were not
  be visible when dumping the pipeline which was very confusing.
* Make encodebin create the right encoder with a capsfilter (and parser)
  to properly enforce the format specified by the user, and so that the
  encoder properties specified in the encoding profile are respected.
* Use `decodebin` to do the decoding instead of selecting a decoder
  ourself and not plug any parser etc...
* Ensure that negotiated format in the sinkpad of smart encoder is fixed
  through time when the user requested a non dynamic output
* Add a parser at the beginning of the smart encoder
* Handle errors when reencoding

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/751>