urisourcebin: Allow more cases for posting stream-collection

Previously urisourcebin only allows stream-collections messages from adaptive demuxers or sources to be posted.

This commit also allows the case where they come from a single parsebin. We still want to prevent it in the case where they are multiple parsebins, since that would require some form of aggregation to show a single/unified collection.

In order to avoid a regression with uridecodebin3 behavior, we also implement support for GST_QUERY_SELECTABLE, so that uridecodebin3 can figure out whether it should let GST_MESSAGE_STREAM_COLLECTION flow upwards (because app/user could react on it) or whether it drops it in order for decodebin3 to do the collection aggregation and posting.