Update Load menu world selection on create/delete

[sparr]

Summary

None

Purpose of change

Creating or deleting a world in the world menu results in the Load menu selecting the wrong world by default for loading

Describe the solution

Move last_world_pos out to the main_menu class, and update it when creating or deleting worlds based on their order

Describe alternatives you've considered

Could pass last_world_pos into world_tab by reference instead of moving it to the class

Testing

Created and deleted worlds and went back and forth between menus

Additional context