Implement strftime-like formatting in fax URLs

This can be used to download faxes where the date of the fax is encoded
into the URL (like surfacewinds_24hr_20170731_213000.gif).

All the standard strftime special sequences are supported, and the
<Map> XML tag can contain the DateOffset attribute in order to specify
an offset (in days) to be added/substracted to the current date.

For example:
  <Server Name="test" Url="http://www.test.net/">
    <Region Name="test">
      <Map Url="%Y%m%d.html" Contents="today" Area="1" />
      <Map Url="%Y%m%d.html" DateOffset="-1" Contents="yesterday" Area="1" />
      <Map Url="%Y%m%d.html" DateOffset="+1" Contents="tomorrow" Area="1" />
      <Area Name="1" lat1="40N" lat2="70N" lon1="50W" lon2="50E" />
    </Region>
  </Server>

src/InternetRetrievalDialog.cpp

@@ -292,8 +292,13 @@ bool InternetRetrievalDialog::OpenXML(wxString filename)
                                 url.Scheduled = false;
                                 url.Server = server.Name;
                                 url.Region = region.Name;
-
-                                url.Url = region_url + wxString::FromUTF8(g->Attribute("Url"));
+
+                                long offset = 0;
+                                wxString s_offset = wxString::FromUTF8(g->Attribute("DateOffset"));
+                                s_offset.ToLong(&offset);
+                                wxDateSpan dateSpan = wxDateSpan::Days(offset);
+                                wxDateTime dateUrl = wxDateTime::UNow().Add(dateSpan);
+                                url.Url = region_url + dateUrl.Format(wxString::FromUTF8(g->Attribute("Url")));
                                 url.Contents = wxString::FromUTF8(g->Attribute("Contents"));
                                 url.area_name = wxString::FromUTF8(g->Attribute("Area"));