Seats support

[Martin Minarik]

Hello.

Multi seat support is welcome. There is already some code
in place.

Ability to define anything in weston.ini file is probably
not needed, because the information can be obtained from
udev.

It happens in udev-seat.c The udev should provide a
ID_SEAT for every device.

Currently, all the devices are usually added to default_seat.

> output (monitors) channels for each of that seats

the outputs (monitors and screens etc) are not related to
seats at all. Every input device can work on any output.

If you have any other question feel free to ask