Make sure we display enabled ListItem on hover

Summary:
I just realized there was a weird workaround in plasma-nm where it gets
checked so it's shown.
Properly use the declarative API instead of listening to a ton of signals.

Test Plan: Tested with plasma-nm and org.kde.plasma.notifications plasmoids

Reviewers: Plasma, mart

Reviewed By: mart

Subscribers: mart, davidedmundson, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4214