Added throbber to library to show progress whilst loading media items

src/Umbraco.Web.UI/umbraco_client/FolderBrowser/Css/folderbrowser.css

@@ -142,6 +142,10 @@
     width: 200px;
 }
 
+.umbFolderBrowser .throbber {
+    margin: 0 20px;
+}
+
 .umbFolderBrowser .items {
     margin: 5px 15px;
     padding: 0;

src/Umbraco.Web/UI/Controls/FolderBrowser.cs

@@ -115,6 +115,9 @@ namespace Umbraco.Web.UI.Controls
             // Create the filter input
             sb.Append("<div class='filter'>Filter: <input type='text' data-bind=\"value: filterTerm, valueUpdate: 'afterkeydown'\" /></div>");
 
+            // Create throbber to display whilst loading items
+            sb.Append("<img src='images/throbber.gif' alt='' class='throbber' data-bind=\"visible: filtered().length == 0\" />");
+
             // Create thumbnails container
             sb.Append("<ul class='items' data-bind='foreach: filtered'>" +
                       "<li data-bind=\"attr: { 'data-id': Id, 'data-order': $index() }, css: { selected: selected() }, event: { mousedown: toggleSelected, contextmenu: toggleSelected, dblclick: edit }\"><div><span class='img'><img data-bind='attr: { src: ThumbnailUrl }' /></span><span data-bind='text: Name'></span></div></li>" +