Verify that headless applications are supported

[mauroservienti]

In Radical 1.x users can boot an application without a main window. This is needed to create applications that for example starts as "hidden" in the tray area and show a window only on user request.

• Validate that Radical 2.0 still supports headless applications
• Validate that Radical 2.0 still supports headless applications when using the generic host