evdev: Port evdev input backend to libinput

Instead of having its own evdev input device processing implementation,
make clutter's evdev backend use libinput to do input device processing
for it.

Two GObject parameters of ClutterInputDeviceEvdev (sysfs-path and
device-path) are removed as they are not used any more.

Before ClutterDeviceManagerEvdev had one virtual core keyboard and one
virtual core pointer device. These are now instead separated into seats,
which all have one virtual core keyboard and pointer device respectively.

The 'global' core keyboard and pointer device are the core keyboard and
pointer device of the first seat that is created.

A ClutterInputDeviceEvdev can, as before, both represent a real physical
device or a virtual device, but is now instead created either via
_clutter_input_device_evdev_new() for real devices, and
_clutter_input_device_new_virtual() for virtual devices.

XKB state and button state is moved to the seat structure and is thus
separated per seat. Seats are not a concept exposed outside of clutter's
evdev backend.

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

https://bugzilla.gnome.org/show_bug.cgi?id=720566