Add 'Account Settings' action to top-level collection context menu

Summary:
Makes accessing account configuration much easier and straightforward. The action
is only displayed in the context menu if the collection is a top-level collection,
so it does not clutter the context menu of regular mail collections.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D14637