Move the loading flag to PanelCtrl (#9929)

public/app/features/panel/metrics_panel_ctrl.ts

@@ -11,7 +11,6 @@ import {metricsTabDirective} from './metrics_tab';
 
 class MetricsPanelCtrl extends PanelCtrl {
   scope: any;
-  loading: boolean;
   datasource: any;
   datasourceName: any;
   $q: any;

public/app/features/panel/panel_ctrl.ts

@@ -31,6 +31,7 @@ export class PanelCtrl {
   containerHeight: any;
   events: Emitter;
   timing: any;
+  loading: boolean;
 
   constructor($scope, $injector) {
     this.$injector = $injector;