Repurpose L/R in RGUI to scroll to next letter.

A common feature in controller-based file selection pickers
is to be able to scroll to next letter in the alphabet.

I've changed L/R in RGUI so in the file selector, it'll scroll to next "letter", which makes it far more convenient to scoll through gigantic folders. The old behavior of L/R (+/- 16 entries) was kinda arbitrary as there's already a similar feature available (left/right dpad).

It'll handle directories and files individually.