[#2496] [GTKUI] Fix updating core_config before setting default options

 * Remove duplicate entry in init.
 * Call update if empty config and prevent potential loop in update method.
 * Ensure that the queue Add button is sensitive, even when automatically adding.
Merge pull request #18 from deluge-torrent/develop

[#2496] [GTKUI] Fix updating core_config before setting default options