Jean-Baptiste Lab

Correct implementation of BluetoothDevice.connect() method to allow user code to catch connection timeout

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This addresses https://github.com/pauldemarco/flutter_blue/issues/299 and probably a few others.

I added mscgen support to the extension (http://www.mcternan.me.uk/mscgen/) which work very similarly to graphviz, feel free to merge this if you like