Add pie charts to promdash

See title. Plz comment with anything I've missed, or additional functionality you would like like.

requested in #114 

@tomcz @juliusv @grobie @ohookins

Doesn't `expression` always get set when creating a new widget (though it's set to an empty string, which might be false here, but then we just set it to an empty string again)? Do we need this if-condition? Maybe we only need it to not refresh a graph with an empty expression, but not to set all expression fields again?