upgrade Bullseye from 3.6.0 to 4.0.0

[adamralph]

I see you're using a few things from the Bullseye.Internal namespace—mainly Palette and HostExtensions. FYI I'm currently working on making those public in Bullseye 4.1.0. One thing I'd like to confirm before releasing 4.1.0 is that the new API will remove the need for you to use the Bullseye.Internal namespace. But to do that, I think it's best if you've already moved to the Bullseye 4.0.0 API, so we can isolate the changes required for you to move from 4.0.0 to 4.1.0.