applets/digital-clock: use ListView for plugins
Reason for the change
The calendar plugin names are not completely self-explanatory; if you don't already know that "Alternate Calendar" shows the date in an alternate calendar format or that "Calendar Events" is for showing events from an Akonadi calendar, it might not be apparent from the (otherwise sensible) names. The plugins already have description fields in their metadata (though the current strings are not quite optimal, as they weren't exposed in the UI) and even icons, but we don't use them.
Instead, use a ListView and (Check)IconTitleSubtitle to display nice- looking list entries for the plugins that can have descriptive labels associated with them.
Test plan
Open Calendar config page
Screenshots or screen recordings
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Bugs fixed
Strings obvously need to be made more informative.