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/stdpaths.h

@@ -75,6 +75,11 @@ public:
 
     // virtual dtor for the base class
     virtual ~wxStandardPathsBase();
+
+protected:
+    // append "/appname" suffix if the app name is set (doesn't append the
+    // slash if dir already ends with a slash or dot)
+    static wxString AppendAppName(const wxString& dir);
 };
 
 #if defined(__WXMSW__)