Populate the target content with source in XLIFF exports

We didn't do it for the previous change of supporting reading from strings but I think this is a perfect use case for using plugin configurations and passing it to getInstance().

This is specific to XLIFF and does not need to result in a change on the data we accept/return.

There have been discussions in the past about also supporting to send an existing translation. While we don't have the API in place for that yet on Job/JobItem/data level, we could already prepare for it by not making it a bool but support multiple values, e.g. 'target' => empty|source|existing_translation or something like that (for now just the first two)

Also wondering about test coverage, we probably want to expose that setting in the file translator too. We could go all the way with configurable/form plugins and embed the selected plugin configuration inside the translator settings. Or we just special case that setting for now. But we could already store the configuration so that we can expand it, e.g. 'format_configuration' => ['translate' => 'source'].

1. +++ b/translators/tmgmt_file/config/schema/tmgmt_file.schema.yml
   @@ -16,3 +16,9 @@ tmgmt.translator.settings.file:
   +      sequence:
   +        type: string
   +        label: Single format configuration option
   

   this isn't a sequence, it is a map of specific keys.

2. +++ b/translators/tmgmt_file/src/FileTranslatorUi.php
   @@ -29,27 +29,45 @@ class FileTranslatorUi extends TranslatorPluginUiBase {
   +    $form['format_configuration']['target'] = [
   +      '#type' => 'select',
   +      '#title' => t('Target translation data'),
   +      '#options' => [
   +        'source' => t('Same as source'),
   +      ],
   +      '#empty_option' => t('Empty'),
   +      '#default_value' => $translator->getSetting('format_configuration.target'),
   +      '#description' => t('Fill the target translation data.'),
   

   title: Target content
   description: Defines what the <target> in the XLIFF file should contain, either empty or the same as the source text.

3. +++ b/translators/tmgmt_file/src/Plugin/tmgmt_file/Format/Xliff.php
   @@ -69,6 +69,20 @@ class Xliff extends \XMLWriter implements FormatInterface {
   +  /**
   +   * {@inheritdoc}
   +   */
   +  public function __construct(array $configuration, $plugin_id, $plugin_definition) {
   +    $this->configuration = $configuration;
   +  }
   

   inheritdoc is wrong because this doesn't implement an interface, so we need a "Creates Xliff instance + params docblock"

4. +++ b/translators/tmgmt_file/src/Plugin/tmgmt_file/Format/Xliff.php
   @@ -141,6 +146,29 @@ class Xliff extends \XMLWriter implements FormatInterface {
   +  /**
   +   * Helper function to write the data.
   +   *
   

   "Helper function" is IMHO a useless prefix, just write what it does: "Writes text according to the XLIFF export settings." ?

+++ b/translators/tmgmt_file/src/FileTranslatorUi.php
@@ -38,13 +38,13 @@ class FileTranslatorUi extends TranslatorPluginUiBase {
       '#default_value' => $translator->getSetting('format_configuration.target'),
-      '#description' => t('Fill the target translation data.'),
+      '#description' => t('Defines what the <target> in the XLIFF file should contain, either empty or the same as the source text.'),
     ] + $xliff_states;

It wasn't visible in my text, but I actually escaped the tag so it is actually visible as one in the description.. this will not actually be visible I think :)

Committed, thanks.