Added plasmoidHeading to clipboard and ported to Page

Summary:
{F8256246}
{F8256249}

Reviewers: #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D29137