Added loading icon to export button of SearchBar

This patch adds an status of loading to the export button of the Search
Bar component. This gives the user feedback that the download initiated
and avoid multiple unecessary download requests.

Fixes: https://gitlab.com/stoqtech/private/bdil/issues/256
Change-Id: Ifaf29b2aa07a3ccd7fdd0afa07241d7d6b4a7b89

search-bar.jsx

@@ -213,12 +213,16 @@ module.exports = SearchBar = React.createClass({
       return null;
     }
 
-    return <div className="input-group-btn">
-      <button name="export-file" className="btn btn-default"
-              title="Exportar relatório" onClick={this._export}>
-          <i className="fa fa-file-excel-o"/>
-      </button>
-    </div>;
+    const iconClasses = this.props.exportLoading ? 'fa fa-refresh fa-spin' : 'fa fa-file-excel-o';
+    const btnClicked = this.props.exportLoading ? null : this._export;
+    return (
+      <div className="input-group-btn">
+        <button name="export-file" className="btn btn-default"
+                title="Exportar relatório" onClick={btnClicked}>
+            <i className={iconClasses}/>
+        </button>
+      </div>
+    );
   },
 
   _getColumnsButton: function() {