To subclass `Component` directly doesn't make any ...

# tornadofx

[edvin]

To subclass Component directly doesn't make any sense, since it can't be used for anything in the framework. The only prototyped component that makes any sense is Fragment. I think I read that in Kotlin 1.1 sealed classes doesn't need to be inside the parent, so for 1.1 I could probably make Component sealed to avoid that confusion.