disable MS-DOS creation for Windows 10 or later

* Microsoft removed the diskcopy.dll that contained the MS-DOS floppy image
  Not a big loss, since we have FreeDOS... ;)
* Closes #545
* Also apply a long overdue refactoring for boot and target types
* Also update the README to add an icon

src/dos.c

@@ -413,9 +413,9 @@ BOOL ExtractFreeDOS(const char* path)
 BOOL ExtractDOS(const char* path)
 {
 	switch(ComboBox_GetItemData(hBootType, ComboBox_GetCurSel(hBootType))) {
-	case DT_WINME:
+	case BT_MSDOS:
 		return ExtractMSDOS(path);
-	case DT_FREEDOS:
+	case BT_FREEDOS:
 		return ExtractFreeDOS(path);
 	}
 	return FALSE;