Change window allocation/destruction to avoid undefined behaviour

Create a new window base class which holds the front/back pointers
and the window class.
This fixes the voluminous warning spam about deleted windows when using
UndefinedBehaviorSanitizer.

src/window_type.h

@@ -755,6 +755,7 @@ enum GameOptionsInvalidationData {
 };
 
 struct Window;
+struct WindowBase;
 
 /** Number to differentiate different windows of the same class */
 typedef int32 WindowNumber;