Rename wxWebRequestEvent::GetResponseFileName() to GetDataFile()

This is shorter and doesn't imply that just the name (and not the full
path) is being returned.

Also rename wxWebResponse::GetFileName() to GetDataFile() for the same
reasons and for consistency. And document this previously undocumented
method.

tests/net/webrequest.cpp

@@ -78,7 +78,7 @@ public:
         case wxWebRequest::State_Completed:
             if ( request.GetStorage() == wxWebRequest::Storage_File )
             {
-                wxFileName fn(evt.GetResponseFileName());
+                wxFileName fn(evt.GetDataFile());
                 REQUIRE( fn.GetSize() == expectedFileSize );
             }
             wxFALLTHROUGH;