Switching TeX engines

Okay, I got rid of the minor modes for XeTeX and Omega.  (To be precise,
the one for Omega is being kept for compatibility reasons for the time
being.)  Instead, there is a variable called `TeX-engine' which can be
used to select the engine.  Since we don't have a way to set stuff for a
document, it is global at the moment but can obviously also be set
file-locally.  The three engine options are currently `default', `xetex'
and `omega'.  One could probably add PDFTeX as well but I didn't see a
need for that because it is already handled by `TeX-DVI-via-PDFTeX'.
The engine can be set through the TeXing options menu or with `M-x
TeX-engine-set <RET>'.

Lemme know what you think.

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Ralf