Add an evaluation bar to the board

[julien4215]

[veloce]

It looks very nice, but I don't think this ought to be part of chessground.

Instead it should be a wrapper widget in the application BoardWithEval or sth like that. I don't think the implementation would be harder if it is external, would it?

[julien4215]

I added it in chessground because of the borderRadius and boxShadow. Since the eval bar is next to the board, it should be applied to both and I can't do that outside of chessground.

[veloce]

I think you can. You need to create another widget that:

• set only the radius to the left side of the board
• set only the radius to the right side of the eval bar
• put board and eval bar side by side

With the board shadow it is more complex, so let's just remove any shadow for that combo widget.

[julien4215]

Done in the PR lichess-org/mobile#944