use fixed-width icons, closes #5555

IPython/html/static/tree/js/notebooklist.js

@@ -204,7 +204,7 b' var IPython = (function (IPython) {'
         item.data('name', name);
         item.data('path', path);
         item.find(".item_name").text(name);
-        item.find(".item_icon").addClass('folder_icon');
+        item.find(".item_icon").addClass('folder_icon').addClass('icon-fixed-width');
         item.find("a.item_link")
             .attr('href',
                 utils.url_join_encode(
@@ -221,7 +221,7 b' var IPython = (function (IPython) {'
         item.data('nbname', nbname);
         item.data('path', path);
         item.find(".item_name").text(nbname);
-        item.find(".item_icon").addClass('notebook_icon');
+        item.find(".item_icon").addClass('notebook_icon').addClass('icon-fixed-width');
         item.find("a.item_link")
             .attr('href',
                 utils.url_join_encode(
@@ -236,7 +236,7 b' var IPython = (function (IPython) {'
 
     NotebookList.prototype.add_name_input = function (nbname, item) {
         item.data('nbname', nbname);
-        item.find(".item_icon").addClass('notebook_icon');
+        item.find(".item_icon").addClass('notebook_icon').addClass('icon-fixed-width');
         item.find(".item_name").empty().append(
             $('<input/>')
             .addClass("nbname_input")