Save/Load dialogs: Allow held directional buttons to scroll up and down the list repeatedly.

The current behaviour does not allow for holding up/down to repeatedly scroll items in save/load lists, which can result in frustration or tired fingers. For the sake of usability, this request intends on letting the list scroll after holding a directional button for ~0.5 seconds (30 frames), at a reasonable rate.

Thanks to @hrydgard for his tip on using modulus to slow down the scrolling (https://github.com/hrydgard/ppsspp/issues/4757).