Add wxDir::GetNameWithSep() and use it to avoid consecutive slashes.

Add wxDir::GetNameWithSep() and use it to avoid consecutive slashes.

It is wrong to use dir.GetName()+"/" to obtain a slash-terminated directory
name as this results in (usually harmless but at best ugly) double slashes at
at the beginning of the string for the root directory. Add GetNameWithSep() to
obtain the correct result in all cases.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71355 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775