Year View Addon not integrated

[MuellerSeb]

I am running Thunderbird 52.7 on Ubuntu 16.04 with Lightning 5.4 and Lightbird 0.7.1.
I was using the Year View Addon (https://github.com/fabrixxm/Year-View), which worked like a charm in Thunderbird, but it doesn't integrate in Lightbird.

Is there a way to make that possible?

Apart from that: Great work!

PS: I also created an issue (fabrixxm/Year-View#20) on Year-View, because I don't know where the problem really is located.