[libinput] Load/store device configuration

Summary:
Device gets a KConfigGroup injected and supports loading device-specific
settings. This is invoked from Libinput::Connection when adding a new
Device.

Whenever a Device option is changed successfully through the DBus
interface it gets synced into the KConfigGroup, thus on next loading of
the Device it gets restored.

The config group follows a pattern of:
[libinput][vendor][product][name]

Thus every device has a specific and persistent configuration.

Test Plan: So far only tested through autotests

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D3264