Use spinbox special value to represent "Use any available port"

WebAPI functionality is preserved (deprecated) for now and should be
removed in the future.

src/base/settingsstorage.cpp

@@ -234,6 +234,13 @@ void SettingsStorage::removeValue(const QString &key)
     }
 }
 
+bool SettingsStorage::hasKey(const QString &key) const
+{
+    const QString realKey = mapKey(key);
+    const QReadLocker locker {&m_lock};
+    return m_data.contains(realKey);
+}
+
 QVariantHash TransactionalSettings::read() const
 {
     QVariantHash res;