parse ExtrasData with QT

[karlicoss]

Here @geek1011 mentioned that they seem to be serialized as QT structures, so hopefully that means the code can be much less fragile by simply following qt spec.

[pgaskin]

Also note that most of the code in my Go QDataStream reader implementation is boilerplate for Go's typing, so doing the same thing in Python should be quite a bit shorter.

If you are fine with a dependency on Qt, you can use PyQt to initialize a QByteArray with the blob, initialize a QDataStream reader on that, and read the QVariantMap from that. From there, you can parse each entry as needed.