Add optionals for new iOS view bindings

[Alex009]

with new ios view bindings required write

let errorText: LiveData<StringDesc> = viewModel.state.error().map { $0 as? StringDesc ?? StringDesc.Raw(string: "") } as! LiveData<StringDesc>
errorLabel.bindText(liveData: errorText)

because optionals not supported

[Alex009]

i think in this issue we should try remove requirement of naming MultiPlatformLibrary for kotlin framework. to do it i suggest: revert action of binding extensions. now we add extension function to UIKit classes and Kotlin/Native can't add it as extension...but when we add extension of kotlin class - it exported correct. so i suggest to change:

fun UIButton.bindEnabled(
    liveData: LiveData<Boolean>,
    enabledColor: UIColor? = null,
    disabledColor: UIColor? = null
) {
    ...
}

to

fun LiveData<Boolean>.bindEnabled(
    button: UIButton,
    enabledColor: UIColor? = null,
    disabledColor: UIColor? = null
) {
    ...
}

[Dorofeev]

In version 0.9.0 new iOS bindings still have same problem. For example this bindings not support optional values

[Alex009]

implemented in #100 and released in 0.9.1