[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.

{F8174617}

Reviewers: #vdg, #plasma, ivan

Reviewed By: #plasma, ivan

Subscribers: plasma-devel

Tags: #plasma

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