delete item checkbox + remove class .odd
implement deleting items
* new file:   ajax/delete-feed-item.php
	ajax php to delete the item

*	modified:   css/screen-details.css
	icon-button is generalized

*	modified:   feed-details.php
	add handling of "deleted" field

*	new file:   js/confirm-dialog.js
	extracted from screens.js

*	modified:   js/feed-details.js
	added the new delete items feature

*	modified:   js/screens.js
	factorized the confirmation dialogs

*	modified:   screens.php
	factorization of the confirmation dialogs

*	modified:   ../lib/css/base.css
	added the icon-button class

*	modified:   ../sql/signage.sql
	add the deleted flag to the feed_contents table