[Primitives:Select] Get rid of Option as the only option

[snatvb]

Hey, I've noticed that Select requires only optional value

I think if I need to have it an optional - I can use Select::<Option<String>.
Also it's really confusing ReadOnlySignal<Option<Option<T>>> type.

For example if I need required option - I can't do it now and I need to use unwrap that move check on runtime instead static.