Provide API to write custom TextInputControls

[FlorianKirmaier]

Some Controls can't implement the public TextInputControl interface. But it would still like to be handled like TextInputControls from JPro.

We could provide an interface that the Control could implement to achieve this.

interface JProTextInputControl {
    void setText(String)
    IntegerProperty selectionStartProperty()
    IntegerProperty selectionEndProperty()
    IntegerProperty caretPosition()
}

class CustomTextInputControl implements JProTextInputControl {
    if(!WebAPI.isBrowser) {
        < add TextInput Shortcuts>
    }
}