Use type aliases FilePath, FilePaths...

... for wxString and wxArrayStringEx, holding file paths (absolute or relative,
directory or plain file); to be replaced later with different types

(not yet using std::vector, becase of some uses of wxArrayString::Index with
two arguments)

src/xml/XMLFileReader.cpp

@@ -38,7 +38,7 @@ XMLFileReader::~XMLFileReader()
 }
 
 bool XMLFileReader::Parse(XMLTagHandler *baseHandler,
-                          const wxString &fname)
+                          const FilePath &fname)
 {
    wxFFile theXMLFile(fname, wxT("rb"));
    if (!theXMLFile.IsOpened()) {