qt5+: Replace `qSort()` with `std::sort()`

[fbriere]

qSort() (along with a good portion of QtAlgorithms) was deprecated in Qt 5.2, in favor of using STL algorithms instead.

Note that std::sort is not an exact drop-in replacement for qSort, due to the following differences:

• qSort can take a Qt container as argument
• qSort will automatically use qLess<T> if defined
• std::sort requires the comparison functor to honour Strict Weak Ordering

The first two points don't apply to us; we never call qSort with a container, nor do we ever specialize qLess<T> anywhere.

As for the last point, since AddressTableModel::sort() eventually delegates its job to the < and > operators (defaulting to false), it can never return true for both a comparison and its opposite, thus satisfying Strict Weak Ordering.

[fbriere]

Note: This was motivated by the fact that qSort() is no longer available in Qt 6 (#297).