Split implementation of keyboard layout handling into a dedicated class

Summary:
So far the implementation of keyboard layout handling was split between
KeyboardInputRedirection and Xkb. KeyboardInputRedirection registered
the global shortcut and did the handling for layout switch and config
changes. Xkb did the notification on layout change.

Layout changes can nowadays be detected through an InputEventSpy. It
can only happen after a key change or an explicit layout switch. Thus
it does not need to be in Xkb anymore which allows to reduce Xkb to
only care about the Xkb keymap and state tracking.

This change introduces a new class KeyboardLayout which is an
InputEventSpy and takes over the task of the layout change notification
from Xkb and the layout management from KeyboardInputRedirection. Thus
everything related to management of keyboard layout is together in one
class.

This allows in future to add unit test to it (requires further cleanup
of Xkb to be able to use it and drop the InputRedirection dependency) and
opens the possibility to also take over keyboard layout management on X11
for the Plasma desktop.

Test Plan: Manual testing

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D4135