Converted to page and added Heading

Summary:
As per D28575, converted the FullRepr to be a Page and added a PlasmoidHeading

Result:

Depends on D28575

Test Plan: Test with D28575

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D28576