DefaultListItemBackground: use outline only to show focus

Part of teams/vdg/issues#32 and
teams/vdg/issues#37.

Currently we use an outline for only the hovered states of these list
items. Unfortunately it is inconsistent with everything:
- In Plasma list items, outlines are used for hover and selection
  states, but not the pressed state. When focus is locked to selection,
  the focused state gets an outline too (since it's using the selection
  state.
- In Menu items, outlines are used for hover and pressed states, which
  are visually identical, and focus is also locked to them.
- In QtWidgets list items, outlines are not used at all (though this is
  something we do eventually want to change, but it's currently
  technically challenging).

It's all a bit of a mess and in need of unification; see the Invent
issues linked above.

This commit moves towards that unification by implementing one of the
things that there was universal agreement on in
teams/vdg/issues#32: that we should use
outlines only to indicate keyboard focus.

Implementation-wise, the outline color now uses the FocusColor from the
theme, as also discussed. And I decided to make the outline thickness
2px rather than 1, since a 1px line was often quite hard to see against
the list item background.