Show system settings window after it's been initialized

System Settings creates two toplevels when it starts on Wayland. The issue seems to be caused by reparenting the main widget of the active view.

When starting, System Settings will show itself to provide feedback, load the view, and reparent its main widget to the stack view.

The problem occurs when reparenting the main widget to the stack view. New RHI code paths will re-create the window handle.

On the other hand, it doesn't look like show() has any practical effect as system settings window is shown and we immediately start loading the view, which will block the main thread. SSDs are common nowadays too so we can neglect plugin loading overhead too.

Therefore this change simply makes system settings window show up once its content item has been initialized.