Add descriptors

[jmlich]

Descriptors are needed to make screenshots on AsteroidOS working - piggz/harbour-amazfish#320

For each characteristics can be available set of descriptors. I decided to add introspect() method into characteristics constructor to enumerate all available descriptors.

Example of use is:

    writeDescriptorAsync("00006002-0000-0000-0000-00a57e401d05", "00002902-0000-1000-8000-00805f9b34fb", QByteArray::fromHex("0100"));

00006002-0000-0000-0000-00a57e401d05 is Content Characteristics from https://wiki.asteroidos.org/index.php/BLE_profiles#Screenshot_Profile

00002902-0000-1000-8000-00805f9b34fb is Client Characteristic Configuration

[piggz]

Do we need to be able to read descriptors also?

[jmlich]

It seems to be easy to add interface for reading of descriptors, but I don't have good use case for that and I haven't done much tests.