Fix in TeX-arg-insert-braces.

* latex.el (TeX-arg-insert-braces): Move
`indent-according-to-mode' after `save-excursion' because
`LaTeX-newline' (used in `save-excursion') deletes trailing
whitespaces.
Set default environment for some class styles.

* style/beamer.el ("beamer"): Set `frame' as default environment.
Set `section' as largest sectioning level.  Add fontification for
`title', `author', and `date'.

* style/letter.el ("letter"): Set `letter' as default environment.
Add some macros and fontification.

* style/slides.el ("slides"): Set `slide' as default environment.