[libinput] Query defaultLeftHanded, save leftHanded, fix ScrollMode config

Some small improvements / fixes to the libinput backend:
- Query libinput_device_config_left_handed_get_default
- Write leftHanded property to config file
- When saving the touchpad scroll mode, write false to all other ones.
  Otherwise it will always enable the last read entry after reboot.
- Use macro for setLeftHanded(bool) and setNaturalScroll(bool)

Reviewers: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D3430