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/arch/LoadingWindow/LoadingWindow_Win32.cpp

@@ -7,6 +7,7 @@
 #include "archutils/win32/WindowIcon.h"
 #include "archutils/win32/ErrorStrings.h"
 #include "arch/ArchHooks/ArchHooks.h"
+#define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #include "CommCtrl.h"
 #include "RageSurface_Load.h"

src/arch/LoadingWindow/LoadingWindow_Win32.h

@@ -4,6 +4,7 @@
 #define LOADING_WINDOW_WIN32_H
 
 #include "LoadingWindow.h"
+#define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 #include "archutils/Win32/AppInstance.h"