nlecomposition: ensure elements pending to be added are not leaked

nlecomposition: ensure elements pending to be added are not leaked

When nlecomposition is finalized with pending add action or io,
associated elements are not unreffed as they should since caller gives
us the reference when calling gst_bin_add causing them to be leaked.
So to make sure we don't leak a reference on element when adding one to
the bin, each stage (action and pending_io) hold a reference on element
and release it when stage is done.

https://bugzilla.gnome.org/show_bug.cgi?id=766455