Introduce wl_resource_queue_event() for sending events later

Some events, such as the display.delete_id, aren't very urgent and we
would like to not always send them immdiately and cause an unnecessary
context switch.  The wl_resource_queue_event() function will place the
event in the connection output buffer but not request the main loop to
poll for writable.  The effect is that the event will just sit in the
output buffer until a more important event comes around and requires
flushing.