Feature: Magento\Backend\Block\Widget\Grid\Column\Renderer

Currency and Price Renderer: use _getValue() to be able to also
retrieve a calculated value instead of a fixed one via getData().

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In Magento\Backend\Block\Widget\Grid\Column\Renderer\Currency the [render](https://github.com/magento/magento2/blob/master/app%2Fcode%2FMagento%2FBackend%2FBlock%2FWidget%2FGrid%2FColumn%2FRenderer%2FCurrency.php#L83) method is not using 
\Magento\Backend\Block\Widget\Grid\Column\Renderer\AbstractRenderer::[_getValue](https://github.com/magento/magento2/blob/master/app%2Fcode%2FMagento%2FBackend%2FBlock%2FWidget%2FGrid%2FColumn%2FRenderer%2FAbstractRenderer.php#L76)(Object $row)

```
    public function render(\Magento\Framework\Object $row)
    {
        if ($data = (string)$row->getData($this->getColumn()->getIndex())) {
```

What is the particular reason for relying on the retrieval method above? ;-)

Would be great if you can implement the `_getValue(`) method as there are use cases for retrieving a computed value.

In `Magento\Backend\Block\Widget\Grid\Column\Renderer::Number` you have the correct useful implementation of `_getValue()` :-)

Thank you for merging!