events: add WIT_EVENT_DEFINE_GLOBAL

WIT_EVENT_DEFINE can't be used outside of some function, because it
uses assert. This macro can and it's sometime usefull to define
an event only once globaly.
tests: events, test WIT_EVENT_DEFINE_GLOBAL
events: make events static

use one event name per file
client: add last_event filed into wit_client_object

There we can store last event which object catched
tests: events, codying style, warrnings, etc.