Consistency in Windows ifdefs/windows.h includes

Changing all defined(_WINDOWS) to defined(_WIN32)

Defining WIN32_LEAN_AND_MEAN in all files except those in Archutils/Win32

src/RageUtil_CharConversions.cpp

@@ -5,9 +5,10 @@
 
 #include <vector>
 
-#if defined(_WINDOWS)
+#if defined(_WIN32)
 
 #include "archutils/Win32/ErrorStrings.h"
+#define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 
 /* Convert from the given codepage to UTF-8.  Return true if successful. */