iconGrid: Rename childrenInRow to columnsForWidth

Since the parameter of the function is the width, reflect that in
the function name. Also, since we are counting columns, not only
children for each row, reflect that in the function name also.

https://bugzilla.gnome.org/show_bug.cgi?id=706081

js/ui/iconGrid.js

@@ -330,7 +330,7 @@ const IconGrid = new Lang.Class({
         return childBox;
     },
 
-    childrenInRow: function(rowWidth) {
+    columnsForWidth: function(rowWidth) {
         return this._computeLayout(rowWidth)[0];
     },