Add a function to retrieve the udev_device handle from a libinput device

The libinput device abstracts a number of things but sometimes the underlying
device is important. The udev device provides the necessary handle to access
that underlying device and various sysfs properties that may be necessary.

A function returning the device node would've done the same thing but is more
prone to race conditions than the udev_device.

https://bugs.freedesktop.org/show_bug.cgi?id=85573

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Add libinput_device_get_context/libinput_seat_get_context

Allow retrieval of the libinput context from the seat and the device.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>