Renamed StateProperty to ObservableProperty and made the protocol public

[MiaKoring]

Like the title says, I changed the protocol to be publicly available, allowing for more third party view update triggering property wrappers, i.e. @Query or @AppStorage

Also I renamed it do ObservableProperty, decoupling it verbally from @State.

Doing this involved declaring the protocol-required properties and functions as public too and replacing StateProperty with ObservableProperty wherever it gets used.

0 sideeffects expected, as there were no logic changes, aside from making some functions/values more accessible.