Add the setting to choose the remote Bluetooth device to connect to

[GrazianoCapelli]

Now the app searches the HC-05 Bluetooth paired device and connects to it. We should give to the user the possibility to change the device, in order to allow to use different BT devices (like HC-06, or BlueSMIRF) or to change the BT device name.

The natural solution is to add a setting with the device selection. Ideally the selection should show the list of the paired BT devices, but we could start with a simpler EditText preference.

[GrazianoCapelli]

Feature implemented (commit 7591793).