<@U25U0KPFT> Demo compiles with `column`. Check th...

# tornadofx

[abhinay]

@carlw Demo compiles with column. Check this:

import tornadofx.*
import java.time.Instant

class RoDemo: View("asdf"){
    override val root = borderpane {
        val data = listOf(Instant.now()).observable()

        tableview(data) {
            column("Millis", Instant::toEpochMilli)
        }
    }
}

which makes me wonder why does it not compile with readonlyColumn?