geom_spoke(): show aesthetics in legend/tooltips

[ASmirnov-HORIS]

By default geom_spoke() does not show aesthetics (angle/radius) in either legend or tooltips.

It would be nice to add default tooltips at least for the radius.

[alshan]

I don't see a legend for "angle" because it's just value without any "scale".

[alshan]

Related: https://github.com/JetBrains/lets-plot/issues/990