Created a top area based on widgets/toparea.svg

Summary:
This add a top area based on the current theme. If the file is not in the current theme, said area is not shown.

Problem: the header is moved to the right by the Svg element, and I'm not sure how to align it correctly anymore.

Depends on D27695

Test Plan:

Reviewers: VDG, Plasma, ngraham

Reviewed By: VDG, ngraham

Subscribers: mart, ngraham, ndavis, plasma-devel

Tags: Plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D27189