Remove TreeNodeBuilderDynatree class and consolidate its methods in TreeNodeBuilder class

#1139
@AparnaKarve : the original idea/design was that each of the functions returns the `@node`. Looking at the `.build` method now, there's a `@node` line at the end. So it does not matter that the original rule is not respected by some methods.
To hold the original logic, I would say that the `@node` statement at the end of `.build` is redundant (as the `@node` would get returned by the previous `case` statement). But you are right that the code now works as expected.