Converted to Page with a PlasmoidHeading in the heading

Summary:
Now the main view is a Page, the toolbar is moved to the heading and it's contained in a PlasmoidHeading

Depends on D28466
Depends on D28575

Test Plan:

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D28467