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:
{F8316475}
{F8316512}

Reviewers: apol, #discover_software_store, #vdg

Subscribers: plasma-devel

Tags: #plasma

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