Window: Add window "token" type, unique for each window instance

Not recycled even for windows with same class/ID
Token may outlive window, unlike pointer

src/window_func.h

@@ -18,6 +18,7 @@
 
 Window *FindWindowById(WindowClass cls, WindowNumber number);
 Window *FindWindowByClass(WindowClass cls);
+Window *FindWindowByToken(WindowToken token);
 Window *GetMainWindow();
 void ChangeWindowOwner(Owner old_owner, Owner new_owner);