Add target attribute to Magento_Ui grid

[thelettuce]

When creating a Magento_Ui grid actionColumn, currently, it is not possible for links to have a target attribute. They can only open in the current browser tab ie. their target is always _self. This change expands that, allowing a target attribute to be added when using the prepareDataSource function of a custom Column module.

Description

added method getTarget to actions.js - it returns "_self" by default or the attribute set when using prepareDataSource.
added code to set link's target attribute to getTarget() in the actions.html template

Code example - when populating the column in a Magento_Ui grid, using a custom class to expand Magento\Ui\Component\Listing\Columns\Column, you can set the target attribute for the links created in your prepareDataSource function, as below.

$item[$name]['edit'] = [
    'href' => $this->_urlBuilder->getUrl(
                          $this->_editUrl,
                            ['id' => $item['id']]
                        ),
                        'label' => __('Edit'),
		'target' => "_blank"
];

Fixed Issues (if relevant)

Manual testing scenarios

1. write module to create a Magento_Ui grid control
2. modify the prepareDataSource function to add a target "_blank" attribute (as the example above)
3. test grid by clicking on link

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)