User event handling with the trait approach

[kchibisov]

Generally one may use an associated type or generic on the event loop, but it'll make API a bit complex and more prone to not being object safe, since the intention is to rely on &dyn T a lot.

I think what we can do is to develop an interface to Wakeup the event loop and then just notify that the loop got woken up, then the users can drain their sources of user events, like channels, Mutex<Vec>, etc.

[kchibisov]

The said approach is implemented with the EventLoopProxy type, so the loop could be woken up.