fix: editorMosaic.layout() called w/ wrong context (electron#752)

Change EditorMosaic.layout to be an arrow function so that the proper
'this' context is baked in. Fixes electron#751

src/renderer/editor-mosaic.ts

@@ -201,7 +201,6 @@ export class EditorMosaic {
     this.customMosaics = this.customMosaics.filter((mosaic) => mosaic !== id);
   }
 
-
   /** Wire up a newly-mounted Monaco editor */
   @action public addEditor(id: EditorId, editor: Editor) {
     const backup = this.backups.get(id);
@@ -247,15 +246,15 @@ export class EditorMosaic {
 
   private layoutDebounce: ReturnType<typeof setTimeout> | undefined;
 
-  public layout() {
+  public layout = () => {
     const DEBOUNCE_MSEC = 50;
     if (!this.layoutDebounce) {
       this.layoutDebounce = setTimeout(() => {
         for (const editor of this.editors.values()) editor.layout();
         delete this.layoutDebounce;
       }, DEBOUNCE_MSEC);
     }
-  }
+  };
 
   public focusedEditor(): Editor | undefined {
     return [...this.editors.values()].find((editor) => editor.hasTextFocus());