Add ical icon support

When you try to add iCal feed to a view that already has an RSS icon, the RSS icon disappears. It took me a while to figure out that this was caused by the Views iCal module that simply resets the iCal icon:

<?php
$this->view->feed_icon = '';
?>

Auch...

I've added a way to add an iCal icon to the feed as well, based on how Views adds the RSS icon. I've included an icon, theme function and hook_theme implementation as well.