Fix accessibility of buttons in the external storage configuration

This means adding the button role and a tabindex attribute,
it also tries to handle the expanded state for the advanced options dropdown.
Note, however, that there is no keyboard only way how to close the dropdown.

apps/files_external/js/settings.js

@@ -715,6 +715,7 @@ MountConfigListView.prototype = _.extend({
 		});
 
 		this.$el.on('click', 'td.mountOptionsToggle>.icon-more', function() {
+			$(this).attr('aria-expanded', 'true');
 			self._showMountOptionsDropdown($(this).closest('tr'));
 		});
 
@@ -1343,6 +1344,7 @@ MountConfigListView.prototype = _.extend({
 			var mountOptions = dropDown.getOptions();
 			$('body').off('mouseup.mountOptionsDropdown');
 			$tr.find('input.mountOptions').val(JSON.stringify(mountOptions));
+			$tr.find('td.mountOptionsToggle>.icon-more').attr('aria-expanded', 'false');
 			self.saveStorageConfig($tr);
 		});
 	}

apps/files_external/templates/settings.php

@@ -173,11 +173,11 @@ function writeParameterInput($parameter, $options, $classes = []) {
 					</td>
 				<?php endif; ?>
 				<td class="mountOptionsToggle hidden">
-					<div class="icon-more" title="<?php p($l->t('Advanced settings')); ?>"></div>
+					<div class="icon-more" title="<?php p($l->t('Advanced settings')); ?>" role="button" tabindex=0 aria-expanded=false></div>
 					<input type="hidden" class="mountOptions" value="" />
 				</td>
 				<td class="save hidden">
-					<div class="icon-checkmark" title="<?php p($l->t('Save')); ?>"></div>
+					<div class="icon-checkmark" title="<?php p($l->t('Save')); ?>" role="button" tabindex=0></div>
 				</td>
 			</tr>
 		</tbody>