Widgets are removed from viewport on server travel

The GameViewportSubsystem removes all widgets from the viewport in UGameViewportSubsystem::HandleRemoveWorld which is bound to the FWorldDelegates::OnWorldBeginTearDown and FWorldDelegates::OnPreWorldFinishDestroy delegates. It's often desirable to display a loading screen when traveling to a new map, but this isn't possible at the moment since any widgets are removed when transitioning from the original map to the transition map.

• Enable seamless travel in the game mode
• In the player controller, add a function to construct a UMG widget outered to the player controller and add it the veiwport
• Play in a stand alone instance
• use the cmd ServerTravel <NewMap> to travel to a different map
• Note that the widget is removed from the viewport