[ESS] default pdf viewer for `ess-swv-PDF'

[Richard M. Heiberger]

On windows the mailcap route gives the wrong answer.
It said
"gv -safer %s"
Presumably this means ghostview, but that is not the name on windows.
The correct name on my system is
c:/Progra~1/Ghostgum/gsview/gsview32
and it isn't in my PATH because the it isn't needed.

The right answer on Windows is to use the native association of file extensions
and call the same system that windows itself uses when the user doubleclicks
a file name in windows explorer.

(w32-shell-execute "open" "final6b.pdf")