[Applet] Port to ExpandableListItem
Summary: Depends on D28033
Test Plan:
All functionality still works. There are minimal visual changes, mostly little things
to make the appearance consistent with other applets using the new ExpandableListItem
component.
Reviewers: VDG, Plasma, drosca, sitter
Subscribers: plasma-devel
Tags: Plasma
Maniphest Tasks: T12812
Differential Revision: https://phabricator.kde.org/D28037