Do not attempt to create an existing directory.

When copying settings from a previous version, the settings sub-directories
may already exist.  The settings initial save code would attempt to create
the sub-directory without checking to see if it already existed which would
trigger a wxWidgets directory already exists warning.

common/settings/settings_manager.cpp

@@ -587,7 +587,8 @@ public:
             path.Replace( m_src, m_dest, false );
             dir.SetPath( path );
 
-            wxMkdir( dir.GetFullPath() );
+            if( !wxDirExists( dir.GetPath() ) )
+                wxMkdir( dir.GetPath() );
 
             return wxDIR_CONTINUE;
         }