sddm-theme: Consistently use the full names of keyboard layouts

There were two inconsistencies:
- The button showed the short name only, which is not user friendly.
  The full list only showed the long names, so it wasn't possible to tell
  what the mapping is.
- The full list was either unsorted or sorted by the short name, which looked
  really messy.

(cherry picked from commit 1f0a7d3a)