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/src/common.h

@@ -0,0 +1,21 @@
+/*
+  Native File Dialog
+
+  Internal, common across platforms
+
+  http://www.frogtoss.com/labs
+ */
+
+
+#ifndef _NFD_COMMON_H
+#define _NFD_COMMON_H
+
+#define NFD_MAX_STRLEN 256
+#define _NFD_UNUSED(x) ((void)x) 
+
+void *NFDi_Malloc( size_t bytes );
+void  NFDi_Free( void *ptr );
+void  NFDi_SetError( const char *msg );
+void  NFDi_SafeStrncpy( char *dst, const char *src, size_t maxCopy );
+
+#endif