(uri)decodebin3: Remove custom force-sw-decoders property

Sadly I wasn't quite paying attention when !485 (merged) went in.

I'd very much like to have that force-sw-decoders property removed from (uri)decodebin3 and instead have a generic way of influencing what elements should be plugged/tried.

Proposal

Add a new signal for selecting a decoder if and only if there is more than one candidate decoder for a given stream.

/**
 * decoder-select
 *
 * Return : NULL to let (uri)decodebin3 do its own selection,
 * or the specific elementfactory from the candidates.
 */
GstElementFactory* decoder-select (GstElement *decodebin3,
                                   GstStream *stream,
                                   GValueArray *candidates)

The default implementation would return NULL to let decodebin3 do its (pre-force-sw-decoder-patch) standard behaviour. And playbin3 can still strip out candidates based on hardware flags, phase of the moon, day of the month or whatever you chose.

Use-cases

• Match ideal decoder based on configured sink (ex: match optimal decoder to configured sink).
• Avoid software decoders
  • FIXME What is the rationale for this use-case ?