MaMuT_: add ENTER key as binding to create a new spot

To facilitate quick annotation of a lineage.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
MaMuT_: add an optional test to prevent new spots to be added within an existing spot.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>