searchDisplay: don't create useless SearchResult's

average constructing time for 1 SearchResult is 3 ms.
It give > 400 result on query like 'a' (on my machine).
https://bugzilla.gnome.org/show_bug.cgi?id=645313

js/ui/iconGrid.js

@@ -281,6 +281,10 @@ IconGrid.prototype = {
         }
     },
 
+    childrenInRow: function(rowWidth) {
+        return this._computeLayout(rowWidth)[0];
+    },
+
     _computeLayout: function (forWidth) {
         let nColumns = 0;
         let usedWidth = 0;