move keyboard to the new approach and refactor the keyboard_interface

- Get rid of the KF5 deprecated methods related to keymap,
  kwayland-server is not source compatible with kwayland, so we don't
  need to keep the deprecated methods
- Move the key repeat, modifiers and keymap handling fully into the
  KeyboardInterface.
- Get rid of some of the keyboard related code base from the
  seat_interface.

Co-Author: Bhushan Shah <bshah@kde.org>