applets/kickoff: put descriptions below titles, as we do elsewhere

Kickoff uses an unusual way of locating list items' descriptions: it
puts them on the same line as the title, aligned to the right.

This saves vertical space but suffers from several issues:

- Because both labels need to inhabit the same line, a long title will
  cause the description to be elided into uselessness
- It's a different style than what we use for other list items with
  fundamentally the same set of data; we don't re-use users' existing
  familiarity with KDE-style list items that typically have the caption,
  subtitle, or description below the label/title
- Saving vertical space is not a win here because we want the applet to
  be touch-friendly, and short list items make that worse, not better

Accordingly, this MR changes the list items to put the descriptions
under the labels to resolve those issues. To avoid the result looking
weird, icon sizes for non-category list items are also increased.
2 jobs for !699 with kickoff-captions-below-list-items in 3 minutes and 10 seconds (queued for 3 seconds)
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