Stabilize listener APIs

[russhwolf]

Looking for user feedback before removing the @ExperimentalListener annotation and treating listeners at the same level of stability as the rest of the library.

• Is the API surface useful as-is, or would it be better to add typed listeners that pass the current value to the callback?
• Is threading a concern? Currently listener APIs do not do any thread-related work and assume clients are correctly doing all interaction from the main thread (in Android and iOS) or can deal with multithreaded usage (in JvmPreferences where listeners are forced to a background thread)
• Are there other usages or issues I haven't considered?

Note one currently-unpublished change which is visible in master: Settings.removeListener() has been replaced with listener.deactivate(). This cleans up the listener internals a little since previously, you could pass a listener to a settings instance which hadn't created it, which might lead to unexpected (usually no-op) behavior.

No specific timeline on this yet, but I want to have an issue to track what work remains before stabilizing

[russhwolf]

I'm planning to add typed listeners as part of 0.7 since there fairly straightforward to include as extension functions on top of the existing API and a bit more natural to use. This will let you do

settings.addIntListener("key") { int -> ... }

instead of

settings.addListener("key") {
    val int = settings.getInt("key")
    ...
}

Still looking for feedback to make sure everything is hitting people's needs before dropping the experimental annotation.

[russhwolf]

Ahead of a 1.0 release, I'm planning to migrate the typed listeners to being part of the ObservableSettings interface rather than being extension functions, and deprecate/remove the untyped addListener(). This will make it easier to align the APIs of ObservableSettings and FlowSettings. I'd then like to drop the @ExperimentalSettingsApi annotations from these APIs in 1.0 which will close this issue.

[russhwolf]

These APIs are no longer marked experimental in 1.0.0-alpha01