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.

original commit: 8cdd106b87b028522ad01a5304709d010db3cb9d

pkgs/gui-pkgs/gui-doc/info.rkt

@@ -13,7 +13,8 @@
                      "syntax-color-lib"
                      "wxme-lib"
                      "gui-lib"
-                     "racket-doc"))
+                     "racket-doc"
+                     "string-constants-doc"))
 (define deps '("base"))
 
 (define pkg-desc "documentation part of \"gui\"")