Simple prefetch functionality (ifvisible.js)

config:
[labs]
allow_prefetch

rainloop/v/0.0.0/app/libraries/RainLoop/Config/Application.php

@@ -229,6 +229,7 @@ Enables caching in the system'),
 				'imap_forwarded_flag' => array('$Forwarded'),
 				'imap_read_receipt_flag' => array('$ReadReceipt'),
 				'smtp_show_server_errors' => array(false),
+				'allow_prefetch' => array(true),
 				'autocreate_system_folders' => array(true),
 				'repo_type' => array('stable'),
 				'custom_repo' => array(''),