AppChooser portal: Overhaul UI

This commit introduces a series of inter-related changes that combine
together to overhaul and modernize the UI, and make it more visually and
functionally consistent with other KDE UIs.

Here are the individual changes:

* Merge the top two pieces of text into one, and use it as the the
  dialog's title
* Move the search field and "show all apps" button to the top, since
  they're both scope selectors
* Make the search field filter all visible items in the view, not just
  non-preferred apps
* Mark the default app using bold text, not a background highlight that
  can mislead you into thinking it has keyboard focus (but it does not)
* When grid delegates are hovered, show a normal-style highlight effect
  rather than changing the cursor shape
* Move the "find more apps" message to the bottom to support a normal
  top-to-bottom flow: first you choose your scope (or not), then you
  look for the app, then if you don't find one, you can use Discover to
  search for more
* Add a placeholder message when the view is empty, either because
  no relevant apps are installed or because no apps matched the user's
  search text