return a configuration when scheduling a state changes to break out of loop

[mockersf]

fixes #1700

State changes have an extra parameter rerun that will keep the current behaviour when true, but exit the current in-frame loop when false, allowing state changes to happen only once per trigger in some case.

I'm not very happy about the rerun name or the doc for it, please help!

All methods scheduling a state change (set, replace, ...) now returns a StateTransitionConfig that can change what happens after the state change:

state.set(AppState::Game).unwrap().end_frame();

[msvbg]

I see you just made some changes, but allow me to bikeshed on the API some more!

1. I think the builder-style pattern here encourages unwrapping, as shown in the example code: state.set(AppState::Game).unwrap().next(Next::NextFrame);.
2. Since this is just a slightly different version of set, can it simply be another function? e.g. set_next_frame. It feels a little bit unnecessary to have a whole config type when there's just one other way of doing it.

[mockersf]

I think the builder-style pattern here encourages unwrapping

I don't really have an issue with unwrap in application code. Also given why you would need this, your game is already crashing 😄
I removed the unwrap in the doc example

Since this is just a slightly different version of set

There are already 8 different methods, adding a variation for each would bump that to 16...

[msvbg]

There are already 8 different methods, adding a variation for each would bump that to 16...

Ah, I didn't read the PR properly. Yeah then it makes a lot more sense.

[NathanSWard]

I'm still not 100% sure I like the name Next for the enum, however I can't seem to find a better name.
So if other people are ok with the name, I'm happy to approve this 🙂

[mockersf]

probably won't be relevant anymore soon