[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/mainWindow.test.js

@@ -14,6 +14,7 @@ import * as Validator from 'common/Validator';
 
 import ContextMenu from '../contextMenu';
 import {isInsideRectangle} from '../utils';
+
 import {MainWindow} from './mainWindow';
 
 jest.mock('path', () => ({