Add and fix things to meet user requirements

[spisladqo]

1. Add still entity (STAND).
   1.5. Add CUSTOM entity. For now, it moves after the player with quite a fast speed.
2. Add function to delete entity by id --- RE_delete_entity.
3. To get entities with their current positions, one can use function get_entities in couple with get_entities_num. Entity with id 0 has index 0, with id 1 --- 1, etc.
4. Now app_run takes check_condition_fun as an argument. It should be a pointer to a user function that checks some condition and returns 1 if lose condition holds, 0 if win condition holds, or any other value if none condition holds.
   4.5. Now app has win_screen_path and lose_screen_path. User should provide them before running app_run. The image that will be shown is decided by check_condition_fun. Refer to the main function for an example.
5. Add function to move entity by id --- RE_move_entity.