LoadingPage: Use Kirigami.Heading

Summary: Unifies the look of the loading message with the other messages in discover.

Test Plan: Font size is correct when installing a package e.g from a flatpak ref.

Reviewers: #discover_software_store, ngraham

Reviewed By: #discover_software_store, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D20675