Form elements and next fuctionality

Hi,
 
  I am not sure if this is a worth while, or it is my specific case. Therefore no need to merge if not a valid contribution, and if needed I will re-write the request, as I did a simple way to show what i am trying to do, and as the elements that use enterkey atom are the following.

./deps/nitrogen_core/src/elements/forms/element_range.erl
./deps/nitrogen_core/src/elements/forms/element_textbox.erl
./deps/nitrogen_core/src/elements/forms/element_inplace.erl

  To explain my pull request, I use tabs to navigate between text boxes (I am not sure if this normal or not), and enter on the final textbox to submit a form. This was working fine before a #link{} added between texbox{} that wasn't filled in. I then check the javascript and found it had only enter as a means to define the next object. Therefore added the ability to use tabs i.e. ASCII code 9.

Simple code to emulate this should be 

[#textbox{id = a, next = b},
 #link{text = "Link"},
 #textbox{id = b, next = okButton},
 #button{id = okButton}]