Retrieve available devices after app did become active

[VNKorchagin]

There are several ways to connect MIDI devices to your phone / tablet:

1. Cable
2. Bluetooth

To connect a MIDI device via Bluetooth, you need to minimize the current application. After the application has become active again, it is necessary to request available devices, otherwise the connected device will be out of available devices.

To get this capability, you need to sign the manager for the UIApplicationDidBecomeActiveNotification

[armadsen]

Fixed by #210.