events: fix counter of emitted events

counter was off-by-one
client: add listeners

Add seat and registry listeners into client's structure and make
wit_client_add_listener public
client: add seat and registry to add_listener, add dbg

add abovementioned interfaces into wit_client_add_listener and add debug
messages
client: make use of wit_client_add_listener

Also it's more consistent when we have these listeners saved in struct
client (it wasn't true before)
client: fixed and extended conditional debug message

the condition was wrong
events: remove dbg message

it had no use anymore