Improve support for Bluetooth devices that send data in binary format

[janosgyerik]

The current app reads from the connected device line by line.
(See BluetoothDeviceConnector class.)
This is not well-suited for devices that send data in binary format.
If newline characters don't appear in the data,
the class might never send a message to the viewer.

Hint: start by implementing a mock device (similar to MockLineByLineConnector),
that reads binary data buffer by buffer, and ideally toggles-hexadecimal view automatically.
(Hexadecimal view is tracked by #5, unfortunately not implemented yet.)