Add the ability to have multiple callbacks in a single listener

[alice-i-cecile]

This feature is necessary because there's currently no way to have more than one of a callback of a single type on an entity.

This is usually just a papercut, but it is required in bevy_mod_picking, in order to merge on_click / on_drag etc into a single callback.

In turn, we want to do that in order to handle the ordering of input events correctly„ using bevyengine/bevy#12100

[alice-i-cecile]

This was discussed with @viridia on Discord: check that conversation for more details and design ideas.

[av1roytman]

Me and Xavier will try and tackle this

[aevyrie]

I had a discussion on discord about the design of what this would look like: https://discord.com/channels/691052431525675048/1038322714320052304/1209594439832567818

The goal is that when you register a callback, you get a handle to that callback, so you can remove or replace it later.

impl<E: EntityEvent> On<E> {
    fn remove<E>(&mut self, handle: ListenerHandle<E>):
    fn replace<E>(&mut self, handle: ListenerHandle<E>, callback: tbd) 
}