Make `On::send_event` more flexible

[alice-i-cecile]

    pub fn send_event<F: Event + From<ListenerInput<E>>>() -> Self {

This is the current signature of On::send_event.

However, I want to be able to quickly do things like "send an AppExit event when this button is clicked".

I propose three related methods:

1. send_event: takes an event of type F and captures it in the closure.
2. convert_and_send_event: current behavior: converts E to F and sends it.
3. send_default_event: sends an event with the Default value.

[alice-i-cecile]

Note that in some cases, I simply can't add a From impl between the event types: orphan rules block implementations on e.g. AppExit.

[alice-i-cecile]

    fn send_default_event<E: Event + Default>() -> Self {
        On::run(|mut event_writer: EventWriter<E>| {
            event_writer.send_default();
        })
    }

A working prototype of 3.

[aevyrie]

We could do all three things with a single closure version:

send_event<F: Event>(impl FnMut(E)-> F>)

1. Capture send_event(move |_| my_other_event.clone())
2. Convert send_event(MyEvent::from)
3. Default send_event(|_| MyEvent::default)