Take advantage of `Into<Option<_>>` args

/!\ check in progress. this includes a branch PATCH commit

Depends on gstreamer-rs!1134 (merged)

Commit 24b7cfc8 applied changes related to nullability as declared by gir. One consequence was that some functions signature ended up requiring users to pass Some(val) when they could use val before.

This commit applies changes on gstreamer-rs which, will honoring the nullability stil allow users to pass val for the few affected functions.

This commit also fixes the signature for Element::request_new_pad which was updated upstream.