game_list: Fix folder reordering

The bug(s) happened because we swapped the contents on values.game_dirs, but the pointer each item had to their respective game_dir wasn't updated. This made it so that the item had the wrong game_dir associated with it after a "move up" or "move down" operation. It can be observed by choosing "open directory location" after such operation.

Changed from raw pointer to an index because it's equivalent but a bit clearer, but the change is not essential.

Co-Authored-By: Vitor K <29167336+vitor-k@users.noreply.github.com>