[MWF] Improve ellipsis handling

- Don't subtract room for ellipsis. Ellipsis should be
  handled by the renderer so we shouldn't reserve space for it.
  Otherwise we end up not getting ellipsis (because the renderer
  thinks there is enough space available), or we don't use up the
  available space.
- Don't allow text to go below button if we want to show ellipsis.

(There will probably be not difference visible when using the Cairo renderer, but it will improve things when using the experimental Pango renderer)