Improve getTotalTrackCount taking into account USB and SD

[ghmartin77]

Introduce different methods for USB and SD:

    uint16_t getTotalTrackCountUsb()
    {
        drainResponses();
        sendPacket(0x47);
        uint16_t result = listenForReply(0x47);
        return result;
    }

    uint16_t getTotalTrackCountSd()
    {
        drainResponses();
        sendPacket(0x48);
        uint16_t result = listenForReply(0x48);
        return result;
    }

For backwards-compatibility implement getTotalTrackCount() based on PlaybackSource set invoking one or the other. Please also see #37.

[Michael Miller]

#40

[Michael Miller]

#41

[Michael Miller]

v.1.0.6