wayland: use static consistently

Declare everything that is only needed inside wayland_common.c as static.
wayland: remove set_user_data from seat_listener

The user_data is passed on add_listener and can later be changed with
set_user_data. But because we don't want to change it later and because it is
the same object remove the set_user_data call.

This might be a copy&paste leftover from the initial draft for the wayland
backend.
wayland: cleanup registry_handle_global

The wl_registry object is already passed as a parameter. No need to create
a temporary variable.