Close the current window instead of the main window from the menu bar

Systems with native menu bars show the same menu bar for all child windows. As a result, you cannot assume that the menu bar's parent (the `MainWindow`) will be the window in focus.

launcher/Application.h

@@ -189,6 +189,7 @@ public slots:
         MinecraftAccountPtr accountToUse = nullptr
     );
     bool kill(InstancePtr instance);
+    void closeCurrentWindow();
 
 private slots:
     void on_windowClose();