Extend xkb support

[nadevko]

1. Add support of ISO_Next_Group and ISO_Prev_Group that can be named NextLayout and PrevLayout. Being able to embed a toggle configuration into a layout is a great idea I think.
2. Variants does't support fullName, while xkb can.
3. If you assign to Return or Backspace level3-key, thay become unrepeatable.

[DreymaR]

If anyone fully understands XKB by-key repeat behavior, please share the knowledge.