#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.
Oh, wait... My IDE doesn't complain (yet) but this shouldn't work when I try to create a Column with `new Column(MD_8)` because then it's not clear which of the varargs constructors (with empty varags value) should be used.