event: Remove EventType enum and pass a callback to event_push

This approach is more flexible because we don't need to support a fixed set of
"event types", any module can push events to be handled in main loop by simply
passing a callback to the Event structure.

src/nvim/os/signal.c

@@ -164,7 +164,7 @@ static void signal_cb(uv_signal_t *handle, int signum)
 
   Event event = {
     .source = signal_event_source(),
-    .type = kEventSignal,
+    .handler = signal_handle,
     .data = {
       .signum = signum
     }