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_opendialog.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_OpenDialog( "png,jpg;pdf", 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;
+}

extern/nfd-modified/test/test_opendialogmultiple.c

@@ -0,0 +1,32 @@
+#include "nfd.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+/* this test should compile on all supported platforms */
+
+int main( void )
+{
+    nfdpathset_t pathSet;
+    nfdresult_t result = NFD_OpenDialogMultiple( "png,jpg;pdf", NULL, &pathSet );
+    if ( result == NFD_OKAY )
+    {
+        size_t i;
+        for ( i = 0; i < NFD_PathSet_GetCount(&pathSet); ++i )
+        {
+            nfdchar_t *path = NFD_PathSet_GetPath(&pathSet, i);
+            printf("Path %i: %s\n", (int)i, path );
+        }
+        NFD_PathSet_Free(&pathSet);
+    }
+    else if ( result == NFD_CANCEL )
+    {
+        puts("User pressed cancel.");
+    }
+    else 
+    {
+        printf("Error: %s\n", NFD_GetError() );
+    }
+
+    return 0;
+}

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;
+}

extern/nfd-modified/test/test_savedialog.c

@@ -0,0 +1,28 @@
+#include "nfd.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+/* this test should compile on all supported platforms */
+
+int main( void )
+{
+    nfdchar_t *savePath = NULL;
+    nfdresult_t result = NFD_SaveDialog( "png,jpg;pdf", NULL, &savePath );
+    if ( result == NFD_OKAY )
+    {
+        puts("Success!");
+        puts(savePath);
+        free(savePath);
+    }
+    else if ( result == NFD_CANCEL )
+    {
+        puts("User pressed cancel.");
+    }
+    else 
+    {
+        printf("Error: %s\n", NFD_GetError() );
+    }
+
+    return 0;
+}