Print stacktrace if the error was critical
Added the UIManager to the abstract class Scene

Every Scene needs its own UI so the UIManager has been removed from the
global access via SGL.Components. The SceneManager now requires a Scene
argument instead of IScene. The Scene will automatically initialize the
UIManager. An UIControl now requires an instance of the assigned
UIManager. For example: public TextBox(UIManager instance) :
base(instance) { }