Add Contracts to DSL Builders

[nbilyk]

Builder:

@ExperimentalContracts
fun Owned.textArea(init: ComponentInit<TextAreaImpl> = {}): TextAreaImpl {
	contract {
		callsInPlace(init, kotlin.contracts.InvocationKind.EXACTLY_ONCE)
	}
	val t = TextAreaImpl(this)
	t.init()
	return t
}

See if there is a way to not need the @ExperimentalContracts annotation.
The documentation mentions the compiler flag: -Xuse-experimental=kotlin.contracts.ExperimentalContracts

[nbilyk]

When we start using Contracts update the Style Guide in the Wiki