Settings API: enhance multiselect fileds

Add optional "select all"/"select none" buttons to multiselect fields,
similarly to what already exists for the country field.

includes/abstracts/abstract-wc-settings-api.php

@@ -666,6 +666,7 @@ abstract class WC_Settings_API {
 			'desc_tip'          => false,
 			'description'       => '',
 			'custom_attributes' => array(),
+			'allbuttons'        => false,
 			'options'           => array()
 		);
 
@@ -688,6 +689,9 @@ abstract class WC_Settings_API {
 						<?php endforeach; ?>
 					</select>
 					<?php echo $this->get_description_html( $data ); ?>
+					<?php if ( $data['allbuttons'] ) : ?>
+						<br/><a class="select_all button" href="#"><?php _e( 'Select all', 'woocommerce' ); ?></a> <a class="select_none button" href="#"><?php _e( 'Select none', 'woocommerce' ); ?></a>
+					<?php endif; ?>
 				</fieldset>
 			</td>
 		</tr>