add ability to select VGM version when exporting

by default it is 1.71 but please tell me if you want this to be changed

src/gui/guiConst.cpp

@@ -70,6 +70,15 @@ const int altValues[24]={
   0, 10, 1, 11, 2, 3, 12, 4, 13, 5, 14, 6, 7, 15, 8, -1, 9, -1, -1, -1, -1, -1, -1, -1
 };
 
+const int vgmVersions[6]={
+  0x150,
+  0x151,
+  0x160,
+  0x161,
+  0x170,
+  0x171
+};
+
 const char* insTypes[DIV_INS_MAX]={
   "Standard",
   "FM (4-operator)",