An OS window whose entire contents are panel UI. It owns the window, its swap chain and the UI
inside it. Its input comes straight from SDL and never touches the engine's input system -
there's no widget toolkit involved anywhere. A popup is one that hangs off another, the way a
menu does - see Editor.PanelWindow.Popup(Editor.PanelWindow,Vector2,System.Boolean).
Editor only. A game has one window and draws its UI into that. The window itself is the engine's
internal SurfaceWindow; this is the public face of it for tools, and lives in Sandbox.Tools so
game code can't open windows.