Add font name completion in fontspec.el.

* tex-style.el (LaTeX-fontspec-arg-font-search): New customizable
variable.
(LaTeX-fontspec-font-list-default): Ditto.

* style/fontspec.el (LaTeX-fontspec-arg-font): New function.
("fontspec"): Use it.

* doc/changes.texi: Mention `LaTeX-fontspec-arg-font-search' and
`LaTeX-fontspec-font-list-default'.