Add inHistory bool and hide heading on inHistory

Summary: This hides the plasmoidHeading on history for not grouped notifications

Test Plan:
After:

Reviewers: broulik, ngraham

Reviewed By: broulik, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D29474