validate: Fix compilation against latest GstValidate
nle: Handle sending SEEK event recursively to our children

Instead of relying on it being implemented in core (as it is currently
not!)
nle: Downgrade some INFO to DEBUG.

+ makes it more pleasant to read logs in info.
pipeline: connect tracks when added, not only on state change.

+ ghost track src pad before calling track added so that
  pipeline has a pad to link.
+ Remove silly comment.