This commit introduces build script configuration for building a shared
library 'libinput.so' containing the evdev input device functionality
from weston.

evdev.c, evdev.h and evdev-touchpad.c are ported to not use the data
structures and API in weston and libwayland-server in order to minimize
dependencies.

The API of filter.c and filter.h are renamed to not include the
'weston_' prefix.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>