[R] Test for pre-existing Win menu or item

[Gordon Smyth]

Many thanks to Hadley Wickham for pointing out that my question had already 
been answered on the R-help list in December, see

https://www.stat.math.ethz.ch/pipermail/r-help/2003-December/041786.html or
http://maths.newcastle.edu.au/~rking/R/help/03b/7525.html

and to Jeff Gentry for pointing out to look out for new functions 
winMenuNames() and winMenuItems() in R 1.9.0.

Gordon