Add libinput support

Supports relative pointer, axis, keyboard and touch events.

libinput support is only available in combination with libudev.
libxkbcommon is required to perform key mapping. For now the
default keymap is used always (selected when building xkbcommon).

[ChangeLog][QtGui] Added a plugin to get mouse, keyboard and touch events
via libinput.

Change-Id: I469d8992c0cd3e79225cefaeb931697baf86a92b
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>