[weston,v2] weston.ini: Add natural scroll support to weston.ini

[Jiayi Zhao]

This adds support for enabling/disabling natural scrolling
via a boolean in weston.ini:

[libinput]
enable_natural_scroll=true

CHANGES:
 - natural_scroll option has been changed to enable_natural_scroll
 - libinput_device_config_scroll_has_natural_scroll() is not longer compared to != 0

Signed-off-by: Jiayi Zhao <jeff.no.zhao at gmail.com>