emacs: use flx-ido
emacs: always save desktop file
emacs: don't use ido for magit completing read

I don't always want to pick an option from the list. eg:

 * when pushing to gerrit, refs/for/branch-name wont be in the list.
 * when trying to view the log of a non-current branch, I need to enter
   an empty string as the second argument to ranged log. but this is
   impossible with ido.
emacs: bind ace-jump-mode to M-j

I don't usually like overriding default bindings, But
comment-indent-new-line is also bound to C-M-j and ace-jump-mode should
really be as quick as possible to invoke.