event: add event hook

event: add event hook
    1. Every modules, to want to handle input events, are use
       pepper_compositor_add_event_hook() to placing event handler, even
       compositor itself
    2. pepper_compositor_add_hook() insert a new hook at the head of hook_list
    3. pepper_seat_handle_event() traversing hook_list from head to tail, and
       call hook_handlers
    4. last installed hook called first.

Change-Id: I6b26754dee0c6129afd67010175df395df67fdd4