[MM-51874] Migrate loading screen to singleton (#2655)

* Migrate loadingScreen to singleton

* REVERT ME when MainWindow singleton changes are merged

* Revert "REVERT ME when MainWindow singleton changes are merged"

This reverts commit 2de5520117b9aefb8eeb161d493de7cb275f7a5b.

src/main/windows/callsWidgetWindow.test.js

@@ -10,6 +10,7 @@ import {
     MINIMUM_CALLS_WIDGET_HEIGHT,
     CALLS_PLUGIN_ID,
 } from 'common/utils/constants';
+
 import WebContentsEventManager from '../views/webContentEvents';
 
 import CallsWidgetWindow from './callsWidgetWindow';