New hooks TeX-after-document-hook and LaTeX-after-usepackage-hook.

* latex.el (TeX-after-document-hook): New hook.
(TeX-arg-document): Use it.
(LaTeX-after-usepackage-hook): New hook.
(LaTeX-arg-usepackage): Use it, remove babel specific code.

* style/babel.el: Update copyright years.
(LaTeX-env-babel-lang): New function.
("babel"): Add `LaTeX-env-babel-lang' to
`LaTeX-after-usepackage-hook'.