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.
I guess it's because of my poor understanding of PPSSPP internals/API.