pepper: Adding pepper_input_device_t to pepper_seat_t

pepper: Adding pepper_input_device_t to pepper_seat_t

An input device represents a physical input device, while objects like
pointer/keyboard/touch of a seat is logical ones. Logical input objects listen
to the events from physical devices and translate them into logical events.
(i.e. coordinate transform)

1. A seat has a single logical pointer/keyboard/touch.

2. Adding a device into multiple seats is allowed.

3. Users can associate input device with seat based on properties of them.
    i.e. seat name, output name

Change-Id: Id1555df66fc64aacf70276923b127549db5cda04