Add a hotkey to swap D-pad and left analog stick keys

Some games use D-pad for in-game movement & menu selections while others use left analog stick for that (leaving D-pad for items usage, map etc routines). Instead of re-defining keys for every game it's better to have a "swap" hotkey to switch between D-pad and left analog stick keys.

This patch is used in ROSA & OpenMandriva linux but likely it's worth of merging upstream.
Hmm, definitely don't like these hard coded numbers.  Should be using the constants.  Actually, not sure why you're even using the `psp_button_names[]` table above?

-[Unknown]