[AUCTeX] automatic local variable insertion on choosing TeX engine

[jfbu]

Hi,

when going to Command -> TeXing Options menu,
and on selecting XeTeX or LuaLaTeX it would
perhaps be nice if a dialog appeared asking
to add a local variable block at end of document.

The analogous happens in some circumstances
where AUCTeX thinks the document should use
one of XeLaTeX or LuaLaTeX (the logic sometimes
fails), but it would be a fortiori even more
useful when the user has gone via the Command
menu.

Regarding this dialog I am sometimes confused that
it asks to choose XeTeX or LuaTeX, which must
be capitalized this way, but the TeX-engine
variable (afaict) will be set-up as "xetex"
or "luatex" without uppercase.

(btw, I wanted to check that but can't trigger the
dialog now from a `\usepackage{fontspec}`
in preamble, so I don't know actually what
triggers that dialog)

Best,

Jean-François