Various fixes on the DOM binding

I talked about those changes with you the other day, so here they are.
Some arguments where missing from the get_element* functions, so I simply added them.

Concerning get_element_by_id, I was unsure the other day if it was able to apply on a element, but I checked it again and it seems (to me) that it only applies on documents, not elements.

I'm maybe wrong, I just followed what was said one this page: https://developer.mozilla.org/fr/docs/DOM/document.getElementById
Since i'm no DOM expert, I prefer being prudent. :-)