Allow having a header in a ListView that is in an OverlaySheet

Summary:
For some reason when there's a header ListView decides to start the items at 0
and the headerItem is placed in the negative space. This makes it odd when we
add anything on the header.
This patch detects the size of the header and compensates it.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D6666