Button missing in Calibre 5.44

[andyyang158]

Calibre 5.44.0
OS: Windows8.1
The plugin is installed normally, but there is no button.

[steinarb]

I'm guessing this is probably caused by the fix to #32

No attempt was made to preserve backwards compatibility.

It might be possible to check out a version from before that change.

There have been AFAICR no other changes than the ones needed to make it run with calibre 6.