Assign element uid to relationship between parent and child

[brunoviana]

Today it's impossible to retrieve and specific child block from a parent using its _uid because there's not an alias linking the child to the parent, so code like below will return anything.

<?php 
$buttonElement = $parent->getButton();

if ($buttonElement):
?>
    <div>
        <?= $parent->getChildHtml($buttonElement[0]['_uid']) ?>
    </div>
<?php endif; ?>

The reason for this is due to the method getChildHtml that calls \Magento\Framework\Data\Structure::getChildId method to retrieve the child block. Below a snippet of this method.

public function getChildId($parentId, $alias)
{
    if (isset($this->_elements[$parentId][self::CHILDREN])) {
        return array_search($alias, $this->_elements[$parentId][self::CHILDREN]);
    }
    return false;
}

This method uses `$alias' to retrieve the child block.

To make it possible the method \Magento\Framework\View\Element\AbstractBlock::append receives a second parameter with this alias and this PR uses this to fix this issue.