[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.

{F8174625}

Reviewers: #vdg, #plasma, jgrulich

Reviewed By: jgrulich

Subscribers: gvgeo, plasma-devel

Tags: #plasma

Maniphest Tasks: T12812

Differential Revision: https://phabricator.kde.org/D28034