[libinput] Change unset of scroll method and acceleration profile

Go for a more intuitive unset behaviour of scroll method and acceleration
profile properties.

Summary:
Unsetting a scroll method should only work, if the scroll method is currently
active. For example when scroll-two-finger is active (i.e. scroll-edge,
scroll-on-button-down is not active):

• unsetting scroll-two-finger should deactivate it and activate no-scroll
• but unsetting for example scroll-edge should _not_ deactivate scroll-two- finger and _not_ activate no-scroll

Reworked setting scroll method auto test in order to test it.

Regarding acceleration profiles: Unsetting one, should always switch to the
other possible one as long as both are supported. In this case
LIBINPUT_CONFIG_ACCEL_PROFILE_NONE is not a valid option in contrast to the
definition of the scroll method enums.

Reviewers: KWin

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D3590