GUI: implement instrument preview in file dialog

SADLY it's just for the built-in file dialog
but i'll think of a solution for the external one

src/engine/platform/arcade.h

@@ -37,7 +37,7 @@ class DivPlatformArcade: public DivDispatch {
       DivMacroInt std;
       unsigned char freqH, freqL;
       int freq, baseFreq, pitch, note;
-      unsigned char ins;
+      int ins;
       signed char konCycles;
       bool active, insChanged, freqChanged, keyOn, keyOff, inPorta, portaPause, furnacePCM, hardReset;
       int vol, outVol;