(svn r2685) -Codechange: Split the music/sound/video drivers into separate files and move them into subfolders.

This results in shorter and hopefully easier to maintain files.
Note: I had to change paths in #include statements of some unrelated files, because I added the ottd base directory to the include path (-I.)

sound/null.c

@@ -0,0 +1,11 @@
+#include "stdafx.h"
+#include "openttd.h"
+#include "sound/null.h"
+
+static const char *NullSoundStart(const char * const *parm) { return NULL; }
+static void NullSoundStop(void) {}
+
+const HalSoundDriver _null_sound_driver = {
+	NullSoundStart,
+	NullSoundStop,
+};