Convert mootools document.id() to agnostic document.getElementId()

### Some infos
###### Mootools document.id() :
The document.id function has a dual purpose: Getting the element by its id, and making an element in Internet Explorer "grab" all the Element methods. It also **serves as a shorthand for document.getElementById()**.
For the full description please read the mootools documentation here: http://mootools.net/docs/core/Element/Element
###### Now about the part with Internet Explorer:
In IE8 Standards mode, getElementById performs a case-sensitive match on the ID attribute only. In IE7 Standards mode and previous modes, this method performs a case-insensitive match on both the ID and NAME attributes, which might produce unexpected results.
For the reference please visit MSDN: http://msdn.microsoft.com/en-us/library/ie/ms536437(v=vs.85).aspx

Another good reference for compatibility is: http://quirksmode.org/dom/core/

![screenshot 2014-10-21 17 15 41](https://cloud.githubusercontent.com/assets/3889375/4719841/c9a95b7a-592c-11e4-92b5-795a13ec3148.png)

###### Conclusion: Since IE8 all browsers give the same response on the method documet.getElementById()

#### Joomla 3.x supports IE>8
1. That makes the usage of this legacy mootools dependent code useless
2. The form validation script is now a jQuery script. So it doesn’t really makes any sense to have mootools code for submitting a form and jquery code to validate it.

Anyhow this PR will not go to the actual behavior, will just make all the document.id() vanilla, since there is absolutely NO compatibility issue.

#### Tests
Because the two methods ARE IDENTICAL it’s more a review for typos, but if the team thinks otherwise then to complete the tests you have to go in every form, button in the back end and check that functionality is still in place.
Now should be fine :)