Make headings of systray and applet merge
Summary:
When the applet is a page with a plasmoidHeading as header, merge it with systray one.
The only big problem is that it looks bad when you open said applet while in sidebar view, see:
I'd like to solve that by removing the sidebar and adding a back button, but that would be for another patch in the future.
Test Plan: To be tested together with D28467
Reviewers: Plasma, VDG, ngraham
Reviewed By: VDG, ngraham
Subscribers: mart, manueljlin, ognarb, ngraham, plasma-devel
Tags: Plasma
Maniphest Tasks: T10470
Differential Revision: https://phabricator.kde.org/D28575