experiments with Qt style sheets

Hi all,

I introduced two experimental options --standard-dark and --alternate-dark
in order to play with Qt style sheets.  Not everything works as documented
by Qt, so it all takes far more time than I originally hoped; the devil is
the details... The 'css' style sheets are in $TEXMACS_PATH/misc/themes and
they are nice in combination with the -r option.

You can play with other look and feels yourself, simply by replacing one of
the two test stylesheets by something you like and relaunching TeXmacs.

Best wishes, --Joris