QtPositioning: fix API review comments

During the API review we've found out that we're missing some important things:

• convert all comparison operators to hidden friends
• convert all streaming operators to hidden friends
• use constexpr and noexcept when possible
• consider adding qHash overloads for value types
• consider adding move constructors and swap() to value types
• consider using Q_DECLARE_SHARED instead of Q_DECLARE_TYPEINFO()
• add virtual setBackendProperty()/backendProperty() to QGeoAreaMonitorSource

Some of the changes are not BC-breaking, so they can be postponed until later releases. But we should at least check if they make sense.