Add OnPress callbacks

[janhohenheim]

Part of #203
Pattern taken from @Freyja-moth. Thanks Freyja!

Alternative to the following:

handle_title_action should be done using observers, rather than a TitleAction component

Relevant discussion.
The TL;DR is that registering a similar OnPress<T: Event>(pub T), which would be need to trigger observers on presses, would require you to register your T in your menu in order to register the actual generic system for your T. Which seems like a weird extra abstraction to add and a potential source of errors, same a when a user forgets to do add_event for EventReaders and EventWriters.
That, or you setup your logic for triggering observers on presses separately for every menu, which seems like a lot of boilerplate for something as trivial as pressing on a button.
Additionally, adding observer structs introduces additional indirection where none is needed.

The ugly reflection stuff is due to bevyengine/bevy#14496.
The need for one-shot systems to be registered at the top of the function will be gone after #223

[janhohenheim]

I also want some feedback from @alice-i-cecile before merging since this is an alternative to her suggestion

[benfrankel]

I like SystemId here because the API is a lot closer to On::<Press>::run from bevy_mod_picking, and the performance impact should be negligible for menu buttons.

[janhohenheim]

Note that a possible extension of this could be an API like On::<Interaction::Press::run(system). I would not do that for the template, since it would be trivial to add for a user themselves and I don't think anything outside of Press will require a callback. Just wanted to point out the possibility.

[alice-i-cecile]

IMO this is more likely to end up using observers, but this is fine until we get a first party solution.

[janhohenheim]

@alice-i-cecile do you mean by that that a player will end up changing it to observers or that a first party solution will use observers? I assume the latter.

[benfrankel]

IMO on_credits etc. names should instead be named based on the action they perform, like enter_credits or exit.