fix layout of overlaysheet
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:
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