[GTKUI] Fix listview error with new config
[#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.
Revert "Fix #2221 : Addtorrentdialog KeyError accessing download_location"

This reverts commit 686fb3184480527d81ecc2e399b24ae676e7a1e8.