Cleaning up lib-src

FileDialog goes into audacity/src/widgets and the mod-* directories go into
audacity/modules.

This leaves nothing in lib-src that isn't a 3rd-party libs or supporting
files.

modules/mod-null/ModNullCallback.h

@@ -0,0 +1,18 @@
+// The following ifdef block is the standard way of creating macros which make exporting 
+// from a DLL simpler. All files within this DLL are compiled with the LIBSCRIPT_EXPORTS
+// symbol defined on the command line. this symbol should not be defined on any project
+// that uses this DLL. This way any other project whose source files include this file see 
+// MOD_NULL_DLL_API functions as being imported from a DLL, wheras this DLL sees symbols
+// defined with this macro as being exported.
+
+
+/* Magic for dynamic library import and export. This is unfortunately
+ * compiler-specific because there isn't a standard way to do it. Currently it
+ * works with the Visual Studio compiler for windows, and for GCC 4+. Anything
+ * else gets all symbols made public, which gets messy */
+/* The Visual Studio implementation */
+#ifdef _MSC_VER
+#define DLL_API _declspec(dllexport)
+#else
+#define DLL_API __attribute__ ((visibility("default")))
+#endif