moved id to position properties;

--- a/src/Pure/General/markup.ML	Thu Jan 03 22:10:52 2008 +0100
+++ b/src/Pure/General/markup.ML	Thu Jan 03 22:25:11 2008 +0100
@@ -96,7 +96,6 @@
 fun markup_int elem prop = (elem, fn i => (elem, [(prop, Int.toString i)]): T);
 
 val nameN = "name";
-val idN = "id";
 
 
 (* kind *)
@@ -111,6 +110,7 @@
 
 val lineN = "line";
 val fileN = "file";
+val idN = "id";
 
 val (positionN, position) = markup_elem "position";