Minor adjustments to the menu screen

I think there's better improvements coming, but I wanted a few more recent since I've come to quite like the feature.

Needs hrydgard/native#59 accepted first.

This:
 * Avoids analyzing prx files (I open these sometimes) - maybe we should make the ISO reader bail on an invalid file instead of "trying anyway."
 * Makes it 12 recent items on Windows and aligns in a grid.
 * Fixes Minis icons (square) being stretched and looking horrid.
 * Shows the Exit button on Windows for #1193.  Just did it always.

-[Unknown]