StatusItemNotifier: fix overlays by name with icons by name

Summary:
Setting an overlay by name results in a QIcon for it created; OTOH,
this icon is never used to create the final image in case the
main/attention icons are set by name too.

Since KIconEngine supports overlays natively, directly pass the list of
overlays (with just one element -- the overlay set) to it.  As result,
main/attention icons by name are created directly with the requested
overlay by name.

Test Plan:
- builds fine
- main/attention icon by name + overlay by name works now, showing the requested overlay
- main/attention icon by name + overlay by pixmap still works as before
- main/attention icon by pixmap is unaffected, no matter whether/which overlay is set
- the system tray icons of juk, and amarok now have the play/pause overlays

Reviewers: apol

Reviewed By: apol

Subscribers: davidedmundson, apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D17983