aggregator: expose API for ignoring inactive pads

An inactive pad is a pad which, in live mode, hasn't yet received a first buffer, but has been waited on at least once.

Exposing API to support this behaviour allows users of aggregator subclasses to request pads, and not start pushing data on those immediately, while avoiding systematic timeouts.

Subclasses must check in explicitly to this behavior, most likely by exposing a user-facing property, and must check whether a pad needs ignoring when aggregating. That is because by design, aggregator subclasses don't get a list of "ready" pads, but instead directly iterate element->sinkpads.