Extend QBluetoothLocalDevice to cover connected devices per adapter

Users want to discover all connected devices per local adapter. It should provide the following elements:

• signal when device dis(connects)
• list of connected devices
• is device currently connected
• list of paired devices (currently only check whether a device is paired is possible -> this assumes that we have the address of the paired device already)

The application wants to use device X. If device is already paired, it could just ask for the list of known devices and uses them. There is no point that every time some application would like to search some device, the device should be put into the discoverable mode application to find it. It can be used just so that if device X is paired or connected, it is used immediately. If not, the developer asks the user to put the device into discoverable pairing mode and pairs it. Next time the application finds it automatically.