Somewhat undo r692 and r693 but replace alt attribute with HTML5-compliant data-attr to fix Issue 222

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@700 eee81c28-f429-11dd-99c0-75d572ba1ddd

editor/svg-editor.js

@@ -347,7 +347,7 @@ function svg_edit_setup() {
   });
 
 	$('.attr_changer').change(function() {
-		var attr = this.getAttribute("alt");
+		var attr = this.getAttribute("data-attr");
 		var val = this.value;
 		var valid = false;
 		if($.inArray(attr, length_attrs) != -1) {