tsdemux: add/remove pads to flowcombiner differently due to streaming nature

tsdemux: add/remove pads to flowcombiner differently due to streaming nature

Due to mpegts streaming nature some pads are created but are only added
later to the element. This can cause a scenario where the first stream
doesn't have an available decoder (while the next ones still pending
would have) and tsdemux will fail with not-linked as the first stream
added wouldn't be linked.

To avoid this tsdemux needs to add pads to the flowcombiner
when they are created instead of only when adding them to the
element.