Prevent duplicates in url list.
Summary:
When not doing that, drag'n'drop-ing an article from the article list
to the browser opened the article twice.
Test Plan: Tested with various RSS feeds dropping to Firefox, no more duplicated tabs
Reviewers: mlaurent, KDE PIM
Reviewed By: mlaurent, KDE PIM
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D24963