Add writable variant iterables

It is currently possible to any QList<T> stored as a QVariant using QSequentialIterable - the same is possible for QHash and QMap via QAssociativeIterable.

It would be nice if we could also write to these containers, i.e. append/insert elements and change the value of exiting elements. An example of how this could look would be (made up names and values):

QVariant variant(QList<int>({1, 2, 3}));
auto writer = variant.value<QSequentialIterableWriter>();
writer.append(QVariant(42));
writer.insert(0, QVariant(10));
writer.set(3, QVariant(0));