moved AppendAppName() from MSW to common code; modified it to not double the trailing slash, if any

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30026 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/stdpaths.h

@@ -28,9 +28,6 @@ public:
     virtual wxString GetPluginsDir() const;
 
 protected:
-    // append "/appname" suffix if the app name is set
-    static wxString AppendAppName(const wxString& dir);
-
     // get the path corresponding to the given standard CSIDL_XXX constant
     static wxString DoGetDirectory(int csidl);
 };