Give all OverlaySheets consistent headers

Summary:
OverlaySheet has a header: property typically used to give the sheet a title. This patch
used it to give all of Discover's OverlaySheets appropriate titles. The review sheet's
top layout is slightly adjusted to make this look good.

Test Plan:

Reviewers: apol, Discover Software Store, VDG

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D29709