#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.
Now I see it, you've changed `setSize()`, too. Not all previous sizes will be removed. This is not correct *imho* since this is an unexpected behavior for a setter method. Setters should always overwrite older values. To add other sizes there's `addSize()`.