[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, ivan

Reviewed By: Plasma, ivan

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28035