Various API cleanups

There are a lot of Qt 6 todo comments in the code base. Each to-be-removed API  must be deprecated by Qt 5.15. Examples are

• What to do about QBluetooth::Security and QBluetooth::AttAccessContraint. Should they be merged? Can we can come up with a more approriate enum?
• QBLuetoothDeviceDiscoveryAgent
  • Remove inquirytype
  • Add more error types
• Remove concept of handle ID from QLECharacteristic & QLEDescriptor -> deprecation required
• Remove/deprecate the various error() signals
  • current system requires extensive use of qOverload (getter and signal have same name)
  • add replacement (errorOccurred or sth similar) to QLEController, QLEService, QBluetoothServer etc
• QBluetoothSocket::Error & QBluetoothSocket::SocketState should lose the value coupling to QAbstractSocket (the class has no QAbstractSocket dependency, they only share QIODevice)
• QBluetoothServer::setSecurityFlags() (same in QBluetoothSocket)
  • investigate a better parameter type as current set is based on old Linux kernel version (which has been replaced)
• QBluetoothUuid:
  • Convert enums to scoped enums
  • Move debug stream operator into cpp, uninline and add EXPORT (obsolete due to conversion to hidden friends)
• QLowEnergyController:
  • remove ctors/make ctors private
• QLowEnergyService: Rename DiscoveringServices -> DiscoveringDetails or DiscoveringService
• Convert all operator== instances to hidden friends