add modified version of Native File Dialog

it will replace portable-file-dialogs on Windows, and perhaps in
the rest of operating systems (maybe not Linux) as well.

extern/nfd-modified/test/test_pickfolder.c

@@ -0,0 +1,29 @@
+#include "nfd.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+
+/* this test should compile on all supported platforms */
+
+int main( void )
+{
+    nfdchar_t *outPath = NULL;
+    nfdresult_t result = NFD_PickFolder( NULL, &outPath );
+    if ( result == NFD_OKAY )
+    {
+        puts("Success!");
+        puts(outPath);
+        free(outPath);
+    }
+    else if ( result == NFD_CANCEL )
+    {
+        puts("User pressed cancel.");
+    }
+    else 
+    {
+        printf("Error: %s\n", NFD_GetError() );
+    }
+
+    return 0;
+}