Add a PGNXML encoder

This is the PGNXML encoder.

+    $this->pgnxml = strtr($this->pgnxml, array("\n" => ''));

That was a truly terrible try... and of course it did not work.

White space fixes...

Added XML namespace to root element.

Test decoding.

Apparently the Symfony XmlEncoder does not handle the namespace. Let's just do that ourselves.

Introduced injection of the Symfony Encoder. (We'll do some mocking in a separate D8-only issue.)
No changes to the test.

#19: pgn-encoder-1959028-19.patch queued for re-testing.

Pushed to 8.x-1.x. Moving to 7.x-1.x.

#1970094: Convert XmlEncoderUnitTest to a UnitTestBase has landed, so now we have a type of test that has an equivalent in Drupal 7.

#26: pgn-encoder-1959028-26.patch queued for re-testing.

Pushed #26 to 7.x-1.x.