EventMgr: Add Dispatch() with handler and args

Allow users to call event_mgr.Dispatch(handler, args) instead of
constructing the Event instance themselves. Deprecate the old API
and replace users.

There's a subtle change that net_done() may be propagated via
auto_publish() now, but that still needs opt-in from script land
and likely no one did that, or else they'd expected to have it
work anyhow.

src/File.cc

@@ -300,8 +300,7 @@ void File::RaiseOpenEvent() {
         return;
 
     FilePtr bf{NewRef{}, this};
-    auto* event = new Event(::file_opened, {make_intrusive<FileVal>(std::move(bf))});
-    event_mgr.Dispatch(event, true);
+    event_mgr.Dispatch(::file_opened, {make_intrusive<FileVal>(std::move(bf))});
 }
 
 double File::Size() {