FactoryLists: rework iterating methods

eg gst::ElementFactory::list_get_elements returns a Vec gst::ElementFactory::list_filter expects a &[] and returns a Vec, that's all a bit suboptimal :)

changed milestone to %0.18.0

The idea would be to expose all these (transfer full) GLists as a new type that is directly around the list and only provides an iterator, and filtering we would then implement directly in Rust without going through C.

makes sense to me :)

added Enhancement label

mentioned in commit slomo/gstreamer-rs@758f41c6

mentioned in merge request !912 (merged)

mentioned in commit slomo/gstreamer-rs@d8350702

mentioned in commit slomo/gstreamer-rs@668b2b3f

closed with merge request !912 (merged)