CookieManager: Add support for selecting more than one cookie at once
Summary:
When manually cleaning cookies, it is very convenient to be able to delete
a bunch of them at once, when we do not want to delete all cookies.
Reviewers: Falkon, drosca
Reviewed By: Falkon, drosca
Subscribers: falkon
Tags: Falkon
Differential Revision: https://phabricator.kde.org/D24967