[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, dantti, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, kde-utils-devel
Differential Revision: https://phabricator.kde.org/D28036