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]
Thus every device has a specific and persistent configuration.