[Request] Add libretro's retropad to Input_Display.lua

[Sonia-7]

This would be a helpful thing to have when you're running games through libretro, since input display doesn't work there at all and it's a feature I use a lot. There's no need to bother with the RetroKeyboard since it has too many inputs and I don't think a lot of games use them. But retropad has only 12 buttons (just like a SNES controller), and being able to display their P1 and P2 inputs would certainly be useful.