Port "Add an item to a list" action to D8

Code for this is up at https://github.com/fago/rules/pull/102

Bit stuck on this one, here's the deets I posted to github:

Hi - am stuck on two issue on this one, just spoke to fago who didn't know what the issue is either so posting here whilst trying to debug too, although may move to something else whilst I'm stuck!

I'm setting a provided value but it says the context is not valid. Also I'm having to force values for 'unique' and 'pos' even though I say they're not required. Here's the output of my test:

There was 1 error:

1) Drupal\Tests\rules\Unit\Action\ListAddItemTest::testActionExecution
Drupal\Component\Plugin\Exception\ContextException: The outputlist provided context is not valid.

/Users/steve/Sites/d8dev/modules/rules/src/Context/RulesContextTrait.php:61
/Users/steve/Sites/d8dev/modules/rules/src/Context/RulesContextTrait.php:50
/Users/steve/Sites/d8dev/modules/rules/src/Context/RulesContextTrait.php:40
/Users/steve/Sites/d8dev/modules/rules/src/Plugin/Action/ListAddItem.php:90
/Users/steve/Sites/d8dev/modules/rules/tests/src/Unit/Action/ListAddItemTest.php:66
phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:179
phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:132

There was 1 failure:

1) Drupal\Tests\rules\Unit\Condition\UserHasEntityFieldAccessTest::testConditionEvaluation
Failed asserting that false is true.

/Users/steve/Sites/d8dev/modules/rules/tests/src/Unit/Condition/UserHasEntityFieldAccessTest.php:123
phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:179
phar:///usr/local/bin/phpunit/phpunit/TextUI/Command.php:132

FAILURES!

ok, think it's fixed now & ready for testing