Added the substeps functionnality that allows to create changes in a single step (see example to see it working)

As the rest of impress, this part only needs html+css skills. There is no need to add any javascript to use it.
Added events handler for the substeps
We may now use :
document.addEventListener("impress:substepenter", myfunction);
document.addEventListener("impress:substepleave", myfunction);
and, to get the name of the substep :
var substep=event.target.getAttribute("data-active-substep");