#152: Ensure at least one ColumnSize must be set

Ensures at least one ColumnSize must be set, both during construction and with setters. Had to remove constructor with the Widget varargs constructor as the signature would be identical to the ColumnSize varargs constructor. Widgets can still be added after construction using the add method.
Nope, not correct what I said (I should read my own, old code more carefully) ;)

`setSize()` will remove all other sizes. So the result would be MD_4 and MD_5. But I still don't understand why `setStyleName()` is required here  because the style name will be added to the column but the previous `setSize()` line.