Added filter to modify current category of product category widget

I'm adding a filter to modify current category. It is useful for situations where you need to change the current category displayed.

Example of use: You need to display only the subcategories of the parent category instead of current category (because it doesn't have subcategories): 
```php
	/* functions.php */
	function change_wc_widget_category( $current_cat ) {
		if (!projectors_is_top_category ($current_cat->term_id)) {
			$current_cat = get_term_by ('id', $current_cat->parent, 'product_cat');
		}
		return $current_cat;
	}
```