Avoid overriding wxDirDialog::GetPath[s]() unnecessarily

Don't duplicate practically the same code in all ports, just add m_paths
itself to the base class. The only drawback of doing this is that it's
unused in the ports not (yet) using it, but this saves enough code in
the aggregate to be worth it.

include/wx/msw/dirdlg.h

@@ -24,18 +24,9 @@ public:
 
     void SetPath(const wxString& path) wxOVERRIDE;
 
-    // can be used only when wxDD_MULTIPLE flag is not set
-    wxString GetPath() const wxOVERRIDE;
-
-    // should be used only when wxDD_MULTIPLE flag is set
-    void GetPaths(wxArrayString& paths) const wxOVERRIDE;
-
     virtual int ShowModal() wxOVERRIDE;
 
 private:
-    // Used for wxDD_MULTIPLE
-    wxArrayString m_paths;
-
     // The real implementations of ShowModal(), used for Windows versions
     // before and since Vista.
     int ShowSHBrowseForFolder(WXHWND owner);