Turn KNewStuffQuick::CommentsModel into a SortFilterProxy for reviews

Summary:
This adds a new property which allows the user to ask the model
to filter its contents by whether or not a comment has a rating
(which means we consider it a review), and optionally to include
the replies to those reviews.

The filtering options are added as contextual actions on the
comments page, so they easily integrate with the Kirigami-esque
navigation methods

Test Plan:
Without patch: No way to only show reviews
With patch: Button(s) appear(s) on the page which shows reviews and comments to allow switching between showing comments, only reviews, or reviews and their replies

Reviewers: Plasma, KNewStuff, Frameworks, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25272