Change the logging system to be per-context

Change the logging system to be per-context

Rather than a single global logging function, make the logging dependent on
the individual context. This way we won't stomp on each other's feet in the
(admittedly unusual) case of having multiple libinput contexts.

The userdata argument to the log handler was dropped. The caller has a ref to
the libinput context now, any userdata can be attached to that context
instead.

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