tocsetter: clean up and update API for refcounted TOCs

tocsetter: clean up and update API for refcounted TOCs

Let's keep it simple for now:

gst_toc_setter_reset_toc() -> gst_toc_setter_reset()
gst_toc_setter_get_toc_copy() -> removed
gst_toc_setter_get_toc() -> returns a ref now
gst_toc_setter_get_toc_entry_copy() -> removed,
    use TOC functions instead
gst_toc_setter_get_toc_entry() -> removed,
    use TOC functions instead
gst_toc_setter_add_toc_entry() -> removed,
    to avoid problems with (refcount-dependent)
    writability of TOC; use TOC functions instead