Support LEDs in Xkb and libinput

Summary:
The Xkb implementation starts to track the state of the LEDs in the
keymap and emits a signal whenever the LEDs change. This signal is
connected to a method in LibInput::Connection which updates the led
state on all devices and uses it to init the state of the led when a new
device gets connected.

BUG: 369214
FIXED-IN: 5.8.2

Test Plan: Connected a keyboard with LEDs and enabled NumLock and ScrollLock.

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2943