#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.
Ok, so when I do

    Column c = new Column(MD_1, MD_2, MD_3);
    c.setSize(MD_4, MD_5);

then the actual size is MD_1, MD_4 and MD_5. That's a bit confusing, don't you think?