(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.)

driver.h

@@ -0,0 +1,11 @@
+#ifndef DRIVER_H
+#define DRIVER_H
+
+void LoadDriver(int driver, const char *name);
+
+bool GetDriverParamBool(const char* const* parm, const char* name);
+int GetDriverParamInt(const char* const* parm, const char* name, int def);
+
+void GetDriverList(char* p);
+
+#endif