add string-constants? and rearrange string-constants pkgs

the rearrangement moves the string constants documentation
into a package over by the string-constants implementation
and adds the string-constants (meta) pkg.
make the 'name field of color scheme be more forgiving
and improve the example in the docs to be more fool-proof
for copy/paste
make the example in the prefs dialog pick up the background color
more accurately
clean up get-current-color-scheme{-name}