#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.
Why not `Column(final ColumnSize size, final Widget... widgets)`?
This should not collide with other varargs constructor because of different types (`ColumnSize` vs `Widget`).