Improve keyboard selection

Merged Nate Graham requested to merge ngraham/xdg-desktop-portal-kde:keyboard-selection into master
  1. Focus the search field by default
  2. Hitting the down arrow key on the search field focuses the view and selects the first item in it
  3. When there's only one item in the view, select it automatically

As a result of the above, it's now possible to easily search for an app and activate it using the keyboard

@teams/usability @sitter

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