Core icons

[husseinalhammad]

It would be very handy to have the core icons documented like the blocks icons. The icons can be used in multiple ways when building Perch apps. Example usage:

$Smartbar->add_item([
    'title' => 'Documents',
    'link'  => $API->app_nav() . '/documents',
    'icon'  => 'core/document',
    'active' => true,
]);