harness: Take ownership of floating references (pads, elements) passed to the harness

Without this bindings get confused about the meaning of references, and we really own these references if they are not already owned by something else.

CC @hgr

This makes ugly work-around like gstreamer-rs!167 (merged) necessary in the Rust bindings, and in runtime-generated bindings like for Python or JS it's simply unusable because of this.