use an initializer list instead of calling insert a million times

Summary:
looks nicer, is probably also more optimized, and allows us to tag the
dict as const.

Test Plan: builds and kaboutdatatest passes

Reviewers: mpyne, kfunk

Reviewed By: kfunk

Subscribers: kfunk, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6674