Cleanup ValueBoxBase and make Input extend ValueBoxBase

I removed the HasEnabled interface and the EnabledMixin from ValueBoxBase because bootstrap does not have .disabled styles for input elements. Therefore it is not necessary to override the existing isEnabled/setEnabled.

When Input extends ValueBoxBase as discussed in #81 you can also remove a lot of redundant code.

I removed HasFormValue because ValueBoxBase implements HasValue<T> and HasFormValue is essentially the same thing.