Redefine (array) converting populator configuration

[jdreesen]

This redefines the property configuration node of #53, because I don't think this config was good UX.

With the poulator type ConvertingPopulator everything is fine, here it is the same as with the converter.<name>.properties config (<target property name> => <source property name>, i.e. user-defined key with user-defined value).

With the poulator type ArrayConvertingPopulator, however, a specific key with a user-defined value could be added (itemProperty => <custom value>). So suddenly two different types of configuration are mixed here, and I don't think we should do that.

My suggestion for the poulator type ArrayConvertingPopulator is therefore that the value can again be an array (which also corresponds to the config we defined in #54). It would then look like this:

Before (as defined in #53):

  populator:
    my.populator:
      poulator: Neusta\ConverterBundle\Populator\ArrayConvertingPopulator
      converter: my.converter
      property:
        address: adresse
        itemProperty: value

Now:

  populator:
    my.populator:
      poulator: Neusta\ConverterBundle\Populator\ArrayConvertingPopulator
      converter: my.converter
      property:
        address:
          source: adresse
          source_array_item: value

The following is still allowed:

  populator:
    my.populator:
      poulator: Neusta\ConverterBundle\Populator\ArrayConvertingPopulator
      converter: my.converter
      property:
        address: adresse

And likewise:

  populator:
    my.populator:
      poulator: Neusta\ConverterBundle\Populator\ArrayConvertingPopulator
      converter: my.converter
      property:
        address: ~

As well as (source and target with same name):

  populator:
    my.populator:
      poulator: Neusta\ConverterBundle\Populator\ArrayConvertingPopulator
      converter: my.converter
      property:
        address:
          source: ~
          source_array_item: value

And (also source and target with same name):

  populator:
    my.populator:
      poulator: Neusta\ConverterBundle\Populator\ArrayConvertingPopulator
      converter: my.converter
      property:
        address:
          source_array_item: value

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Since the use of other populators (apart from ConvertingPopulator and ArrayConvertingPopulator) was not supported anyway, I also limited the config to these two types.