lisp/org.el: Fix refile targets when using 'full-file-path

* lisp/org.el: (org-refile-get-targets): Fix refile targets when using
  'full-file-path.

When `org-refile-use-outline-path' is set to 'full-file-path the file
target is no longer accessible.

TINYCHANGE
lisp/org.el: Add missing `regexp-quote'

* lisp/org.el (org-link-search): Search string is not quoted. Unable to
  jump to a location containing special characters.

TINYCHANGE
complete with helm is now possible
lisp/org.el (org-icompleting-read): require `helm-mode' because
`helm-completing-read-default-1' is not autoloaded.