playbin3: more flexible URI property

playbin3 currently implements the same URI properties as playbin:

uri: URI of the media to play
suburi: Optional URI of a subtitle
current-uri: The currently playing URI
current-suburi: The currently playing URI of a subtitle

suburi can actually be used to play audio or video, at least with playbin, see #690 (closed), no the naming isn't great.

Also, one may want to play 3 streams together (audio, video, text) which is currently not possible. Or even be able to quickly switch between different streams of the same type.

I was thinking we could implement something more flexible and generic using the new stream APIs:

add a new uris property taking an array of URIs
expose them all using GstStreamCollection
activate by default the first audio/video/text streams
let user activate specific streams using GST_EVENT_SELECT_STREAMS
add a new current-uris property containing the subset of uris currently activated.

@bilboed : any thoughts?