Add filter to allow easier definition of widget background style options

[benlk]

https://github.com/INN/largo/blob/512da701664b329f2f92244bbe54880a6e146431/inc/widgets.php#L144-L146

The new version would look something like:

$options = array(
  	'default' => __( 'Default', 'largo' ),
  	...
);
/* docblock comment goes here */
$options = apply_filter( "largo_widget_background_options", $options );
foreach ( $options as $class => $value ) {
  	?>
  	<option <?php selected( $instance['widget_class'], $class ); ?> value="<?php echo esc_attr( $class ); ?>"><?php esc_html_e( $value ); ?></option>
  	<?php
}

This will make client work like INN/umbrella-citylimits#77 easier.