QLowEnergyController - connectTo/disconnectFrom/Device (OS X, iOS)

Implement connect to device/disconnect from device methods,
add CentralManager class dealing with CBCentralManager (with
additional external logic).
- Improve error handling while in 'connecting' state.
- Use the proper error (ConnectionError) instead of UnknownError +
  make the error handling more uniform while isConnecting == true.
- Use isNull on QBluetoothUuid, no need in ugly 'hasUuid'.

Change-Id: I84a704a746b4677ccb870b9c132db5f7359030c6
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>