Platform-Specific Enum Variants

From https://github.com/rust-windowing/winit/pull/3056#issuecomment-1695542094:

If we want to continue using the extension trait model for platform-specific features, I came up with the following idea that we could use for enums:

enum Event {
    EventA,
    EventB(String),
    EventC {
        data: u16,
    },
    Platform(PlatformEvent),
}

impl PlatformEventExtWayland for PlatformEvent {
    fn event(&self) -> WaylandEvent {
        self.0
    }
}

// Not really needed, just a neat idea.
impl PlatformEventExtWindows for PlatformEvent {
    fn event(&self) -> ! {
        match self.0 { }
    }
}

Alternatively we can always mark the enum #[non_exhausive] and add cfg-gated variants.

For consistency with the current API I would like to go with the first solution. The first thing I would like to apply this to is to re-introduce the ControlFlow enum after #3056.

Start by reading the discussion in PR #3056 and the current API around ControlFlow. Define how the proposed platform-specific enum extension model should reintroduce ControlFlow consistently across platforms; done means the API design is agreed and the implementation covers the relevant platform variants.