Port to use level 1 Heading instead of Title

Summary:
PlasmaExtras.Title is literally identical to a level 1 Heading. Let's port to that
so we have less code to maintain and fewer things to change the next time we want to
adjust the look of titles

Test Plan: No visual changes

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27348