libinput: 1.5.1 -> 1.7.2

Changelogs:

1.5.2: https://lists.freedesktop.org/archives/wayland-devel/2016-November/031926.html
1.5.3: https://lists.freedesktop.org/archives/wayland-devel/2016-December/032110.html
1.5.5: https://lists.freedesktop.org/archives/wayland-devel/2017-January/032610.html
1.6.0: https://lists.freedesktop.org/archives/wayland-devel/2017-January/032746.html
1.6.1: https://lists.freedesktop.org/archives/wayland-devel/2017-February/032940.html
1.6.2: https://lists.freedesktop.org/archives/wayland-devel/2017-February/033169.html
1.6.3: https://lists.freedesktop.org/archives/wayland-devel/2017-March/033377.html
1.7.0: https://lists.freedesktop.org/archives/wayland-devel/2017-March/033531.html
1.7.1: https://lists.freedesktop.org/archives/wayland-devel/2017-April/033962.html
1.7.2: https://lists.freedesktop.org/archives/wayland-devel/2017-May/034037.html

Lots of fixes, most notably around trackpad support on high-dpi
systems.