Summary:
makes the overlaysheet howit's supposed to be rendered:
since it's "global" in the app, it should be parented to the
applicationwindow overlay, and have a "non 100%" width, but something
based upon readability

Test Plan: {F7774628}

Reviewers: #discover_software_store, apol

Reviewed By: #discover_software_store, apol

Subscribers: apol, ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25430